Integrate Razor LanguageServer (without plugin) into src/Razor.

- Added tests for both the language server and the common language server projects.
- Updated publish MSBuild bits to publish to `artifacts/LanguageServer/$(Configuration)/TFM`
- Updated the language server to be netcoreapp3.0

aspnet/AspNetCoredotnet/aspnetcore-tooling#13494
\n\nCommit migrated from 984c638b37

src/Razor/Microsoft.AspNetCore.Razor.Language/src/DefaultRazorIntermediateNodeLoweringPhase.cs

@@ -1099,8 +1099,6 @@ namespace Microsoft.AspNetCore.Razor.Language
 
                 if (node.Source != null)
                 {
-                    Debug.Assert(node.Source.Value.FilePath != null);
-
                     node.Source = new SourceSpan(
                         node.Source.Value.FilePath,
                         node.Source.Value.AbsoluteIndex,