Include Roslynator.Analyzers and Roslynator.CodeFixes with more strict default severity levels

Introduce https://github.com/JosefPihrt/Roslynator, which has 500+ analyzers, and also a large number of refactorings.

Lists of each:

http://pihrt.net/Roslynator/Analyzers
http://pihrt.net/Roslynator/Refactorings

Here's a very preliminary set of some recommendations for default severities. This is by no means exhaustive and is only the set that I came up with upon looking at the Info level severity issues from a single project I'm working on at the moment.

For some of these that aren't already covered, corresponding rules should be added to the C# standards.

``` editorconfig
# RCS1001: Add braces (when expression spans over multiple lines).
dotnet_diagnostic.RCS1001.severity = error

# RCS1003: Add braces to if-else (when expression spans over multiple lines).
dotnet_diagnostic.RCS1003.severity = error

# RCS1007: Add braces
dotnet_diagnostic.RCS1007.severity = error

# RCS1044: Remove original exception from throw statement
dotnet_diagnostic.RCS1044.severity = error

# RCS1123: Add parentheses according to operator precedence.
dotnet_diagnostic.RCS1123.severity = error

# RCS1037: Remove trailing white-space.
dotnet_diagnostic.RCS1037.severity = none

# RCS1036: Remove redundant empty line.
dotnet_diagnostic.RCS1036.severity = none

# RCS1170: Use read-only auto-implemented property.
dotnet_diagnostic.RCS1170.severity = warning

# RCS1213: Remove unused member declaration.
dotnet_diagnostic.RCS1213.severity = warning
```

Start by reviewing the repository's existing C# standards and editorconfig configuration, then compare them with the listed Roslynator analyzers and refactorings. Done means Roslynator is included and the selected RCS rules have documented default severities, with corresponding additions to the C# standards where needed.

Written by the indexing model from the issue text.