Add ability to suppress token substitution in launch settings strings

(Reported via email.)

When launching the debugger, some values in launch profiles have tokens substituted. There are two forms of this:

1. Environment variables which appear as `%MY_ENV_VAR%`.
2. MSBuild properties which appear as `$(MyProperty)`.

It's currently not possible to suppress this behaviour, if you want the value to be passed through unmodified.

---

For example, the following launch profile:

```json
{
"profiles": {
"MyProject": {
"commandName": "Project",
"commandLineArgs": "$(foo.bar)"
}
}
}
```

If you want the literal value `$(foo.bar)` to be passed verbatim to the launched executable, there is no way to achieve that today.

---

The class [`DebugTokenReplacer`](https://github.com/dotnet/project-system/blob/eb4e01a2d65fae7b0d466718356e7b9c5a175dcd/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Debug/DebugTokenReplacer.cs) is responsible for this substitution. Any fix to this issue is likely to apply to that class.

Start with src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Debug/DebugTokenReplacer.cs, which handles substitution for launch profile strings. Trace how %MY_ENV_VAR% and $(MyProperty) values are processed, then define a way for a launch profile value such as $(foo.bar) to be passed verbatim to the launched executable.

Written by the indexing model from the issue text.