Bring back Go to Def, FAR and hover for components via Roslyn

[davidwengier]

Fixes #12276 again
Fixes https://devdiv.visualstudio.com/DevDiv/_workitems/edit/2633418

Commit-at-a-time is thoroughly recommended.

Commits 1-4 expose a DocumentExcerpt service that Roslyn can use to provide nice results in FAR etc., because without the code-gen changes, the generated code is pretty ugly.
Commit 5 "reverts" #12486, bringing #12287 back but without any changes to the code-gen this time, just a new source mapping.
Commit 6 is test baseline updates (only source mappings, no .cs files)
Commit 7 removes the requirement for line pragmas to exactly match source mappings. I left the rest of the validation though
Commit 8 bumps us to a new Roslyn with the new document excerpt services.

[davidwengier]

This line is the only one that is different from #12287. In that PR, it was BuildEnhancedLinePragma:

razor/src/Compiler/Microsoft.CodeAnalysis.Razor.Compiler/src/Language/Components/ComponentNodeWriter.cs

Lines 574 to 577 in d8cb5e5

	using (context.BuildEnhancedLinePragma(node.StartTagSpan, offset))
	{
	context.CodeWriter.Write(nonGenericTypeName);
	}