Escape sequences don't get highlighted when semantic highlighter is enabled

## Issue Description ##
Escape sequences aren't highlighted when semantic highlighting is enabled.
## Steps to Reproduce ##
Write a string or a char with an escape sequence in a C# file, e.g. `"Hello World\n"` or `'\t'`.
## Expected Behavior ##
My theme should highlight the escape sequence white
## Actual Behavior ##
the escape sequence has the same colour as the rest of the string.
## Logs ##

### OmniSharp log ###
Starting OmniSharp server at 3.6.2020, 12:14:03
Target: d:\\source\repos\Penguor\PenguorCS\PenguorCS.sln

OmniSharp server started.
Path: c:\Users\\.vscode\extensions\ms-dotnettools.csharp-1.22.0\.omnisharp\1.35.2\OmniSharp.exe
PID: 1348

[info]: OmniSharp.Stdio.Host
Starting OmniSharp on Windows 6.2.9200.0 (x64)
[info]: OmniSharp.Services.DotNetCliService
DotNetPath set to dotnet
[info]: OmniSharp.MSBuild.Discovery.MSBuildLocator
Located 1 MSBuild instance(s)
1: StandAlone 16.4 - "c:\Users\\.vscode\extensions\ms-dotnettools.csharp-1.22.0\.omnisharp\1.35.2\.msbuild\Current\Bin"
[info]: OmniSharp.MSBuild.Discovery.MSBuildLocator
MSBUILD_EXE_PATH environment variable set to 'c:\Users\\.vscode\extensions\ms-dotnettools.csharp-1.22.0\.omnisharp\1.35.2\.msbuild\Current\Bin\MSBuild.exe'
[info]: OmniSharp.MSBuild.Discovery.MSBuildLocator
Registered MSBuild instance: StandAlone 16.4 - "c:\Users\\.vscode\extensions\ms-dotnettools.csharp-1.22.0\.omnisharp\1.35.2\.msbuild\Current\Bin"
CscToolExe = csc.exe
CscToolPath = c:\Users\\.vscode\extensions\ms-dotnettools.csharp-1.22.0\.omnisharp\1.35.2\.msbuild\Current\Bin\Roslyn
MSBuildExtensionsPath = c:\Users\\.vscode\extensions\ms-dotnettools.csharp-1.22.0\.omnisharp\1.35.2\.msbuild
MSBuildToolsPath = c:\Users\\.vscode\extensions\ms-dotnettools.csharp-1.22.0\.omnisharp\1.35.2\.msbuild\Current\Bin
[info]: OmniSharp.Cake.CakeProjectSystem
Detecting Cake files in 'd:\\source\repos\Penguor\PenguorCS'.
[info]: OmniSharp.Cake.CakeProjectSystem
Could not find any Cake files
[info]: OmniSharp.MSBuild.ProjectSystem
Detecting projects in 'd:\\source\repos\Penguor\PenguorCS\PenguorCS.sln'.
[info]: OmniSharp.MSBuild.ProjectManager
Queue project update for 'd:\\source\repos\Penguor\PenguorCS\Penguor\Penguor.csproj'
[info]: OmniSharp.MSBuild.ProjectManager
Queue project update for 'd:\\source\repos\Penguor\PenguorCS\Test\Test.csproj'
[info]: OmniSharp.Script.ScriptProjectSystem
Detecting CSX files in 'd:\\source\repos\Penguor\PenguorCS'.
[info]: OmniSharp.Script.ScriptProjectSystem
Could not find any CSX files
[info]: OmniSharp.WorkspaceInitializer
Invoking Workspace Options Provider: OmniSharp.Roslyn.CSharp.Services.CSharpFormattingWorkspaceOptionsProvider, Order: 0
[info]: OmniSharp.MSBuild.ProjectManager
Loading project: d:\\source\repos\Penguor\PenguorCS\Penguor\Penguor.csproj
[info]: OmniSharp.WorkspaceInitializer
Invoking Workspace Options Provider: OmniSharp.Roslyn.CSharp.Services.RenameWorkspaceOptionsProvider, Order: 100
[info]: OmniSharp.WorkspaceInitializer
Invoking Workspace Options Provider: OmniSharp.Roslyn.CSharp.Services.ImplementTypeWorkspaceOptionsProvider, Order: 110
[info]: OmniSharp.WorkspaceInitializer
Invoking Workspace Options Provider: OmniSharp.Roslyn.CSharp.Services.BlockStructureWorkspaceOptionsProvider, Order: 140
[info]: OmniSharp.WorkspaceInitializer
Configuration finished.
[info]: OmniSharp.Stdio.Host
Omnisharp server running using Stdio at location 'd:\\source\repos\Penguor\PenguorCS' on host 13260.
[info]: OmniSharp.MSBuild.ProjectManager
Successfully loaded project file 'd:\\source\repos\Penguor\PenguorCS\Penguor\Penguor.csproj'.
[info]: OmniSharp.MSBuild.ProjectManager
Adding project 'd:\\source\repos\Penguor\PenguorCS\Penguor\Penguor.csproj'
[info]: OmniSharp.MSBuild.ProjectManager
Loading project: d:\\source\repos\Penguor\PenguorCS\Test\Test.csproj
[info]: OmniSharp.MSBuild.ProjectManager
Successfully loaded project file 'd:\\source\repos\Penguor\PenguorCS\Test\Test.csproj'.
[info]: OmniSharp.MSBuild.ProjectManager
Adding project 'd:\\source\repos\Penguor\PenguorCS\Test\Test.csproj'
[info]: OmniSharp.MSBuild.ProjectManager
Update project: Penguor
[info]: OmniSharp.MSBuild.ProjectManager
Update project: Test

### C# log ###
empty

## Environment information ##

**VSCode version**: 1.45.1
**C# Extension**: 1.22.0

Dotnet Information
.NET Core SDK (gem�� "global.json"):
Version: 3.1.300
Commit: b2475c1295

Laufzeitumgebung:
OS Name: Windows
OS Version: 10.0.19041
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\3.1.300\

Host (useful for support):
Version: 3.1.4
Commit: 0c2e69caa6

.NET Core SDKs installed:
3.1.100 [C:\Program Files\dotnet\sdk]
3.1.300 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.All 2.1.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 3.1.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.1.0 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 3.1.4 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

To install additional .NET Core runtimes or SDKs:
https://aka.ms/dotnet-download

Visual Studio Code Extensions

|Extension|Author|Version|
|---|---|---|
|abnf|forivall|1.0.0|
|auto-rename-tag|formulahendry|0.1.3|
|bash-ide-vscode|mads-hartmann|1.10.2|
|beautify|HookyQR|1.5.0|
|better-comments|aaron-bond|2.0.5|
|bracket-pair-colorizer-2|CoenraadS|0.1.4|
|code-settings-sync|Shan|3.4.3|
|code-spell-checker|streetsidesoftware|1.9.0|
|colonize|vmsynkov|2.2.2|
|csharp|ms-dotnettools|1.22.0|
|csharpextensions|jchannon|1.3.1|
|docomment|k--kato|0.1.11|
|dotnet-test-explorer|formulahendry|0.7.2|
|EditorConfig|EditorConfig|0.15.1|
|errorlens|usernamehw|3.1.1|
|gc-excelviewer|GrapeCity|2.1.35|
|git-graph|mhutchie|1.23.0|
|gitignore|codezombiech|0.6.0|
|gitlens|eamodio|10.2.1|
|js-debug-nightly|ms-vscode|2020.6.208|
|language-haskell|justusadam|3.2.1|
|language-x86-64-assembly|13xforever|2.3.0|
|markdown-all-in-one|yzhang|3.0.0|
|material-icon-theme|PKief|4.1.0|
|msbuild-project-tools|tintoy|0.3.6|
|npm-intellisense|christian-kohler|1.3.0|
|open-in-browser|techer|2.0.0|
|output-colorizer|IBM|0.1.2|
|path-autocomplete|ionutvmi|1.13.6|
|project-manager|alefragnani|11.0.1|
|python|ms-python|2020.5.80290|
|restructuredtext|lextudio|128.0.0|
|rewrap|stkb|1.11.1|
|search-node-modules|jasonnutter|1.3.0|
|sort-lines|Tyriar|1.9.0|
|TabOut|albert|0.1.6|
|tinypng|andi1984|0.3.3|
|todo-tree|Gruntfuggly|0.0.176|
|turbo-console-log|ChakrounAnas|1.4.0|
|vsc-material-theme|Equinusocio|32.6.0|
|vscode-colorize|kamikillerto|0.8.17|
|vscode-counter|uctakeoff|1.3.5|
|vscode-csharp-snippets|jorgeserrano|0.3.1|
|vscode-eslint|dbaeumer|2.1.5|
|vscode-html-css|ecmel|0.2.3|
|vscode-jumpy|wmaurer|0.3.1|
|vscode-markdownlint|DavidAnson|0.36.0|
|vscode-npm-script|eg2|0.3.12|
|vscode-nuget-package-manager|jmrog|1.1.6|
|vscode-solution-explorer|fernandoescolar|0.3.10|
|vscode-wakatime|WakaTime|4.0.0|
|vscode-xml|redhat|0.11.0|
|vscode-yaml|redhat|0.8.0|;