Editorconfig improvements - do not lose state, trigger re-analysis on change

[filipw]

1. at the moment we set brand new CompilationOptions based on the csproj file when a project update is triggered (e.g. rebuild, change in csproj). This causes us to lose the editorconfig state because it is kept in SyntaxTreeOptionsProvider which gets reset then. The PR ensures the new compilation options are created only when necessary, and also derived from the previous ones.

• fixes Omnisharp randomly ignores editorconfig settings, requires restart dotnet/vscode-csharp#4184
• fixes EditorConfig: .NET ErrorCodes have no effect dotnet/vscode-csharp#4165
• fixes OmniSharp seems to randomly forget EditorConfig state #1955

2. when editorconfig file changes, even though we watch for changes, there is currently no event raised that would trigger a re-analysis. This PR introduces new targeted handling of editorconfig - if an editorconfig file changes, we raise a relevant workspace event (AnalyzerConfigDocumentChanged) which is now newly handled to trigger re-analysis.

• fixes Watch .editorconfig for changes #1904

[ffMathy]

Wow, this is great! Can't wait until this is released. Nicely done!

[filipw]

this PR requires this change to ship with the C# Extension dotnet/vscode-csharp#4230
I merged it already but just for the record some cases may not work (e.g. using pre-release OmniSharp server with a current version of the extension)

[JoeRobich]

LGTM