Improve Razor startup allocations

[ryanbrandenburg]

Currently a large Razor project takes about 30 seconds to be fully available. That's partially out of our control, but we also allocate a lot (5GB total was allocated in my test of a large app), and could likely cut that time significantly by improving allocations. Here are some of the identified painpoints:

Parent	Size	Cause	Note
CSharpSyntaxTree.IsGeneratedCode	140.4MB	StringBuilder operations inside TryUnescapeSectionName	dotnet/roslyn#54511
WorkspaceAnalyzerConfigOptionsProvider.GetOption	213.9	StringBuilder operations inside TryUnescapeSectionName	dotnet/roslyn#54511
Tokenizer.EndToken	147.8MB	Use string pooling?
Lexer.LexSingleDirective	823.0MB	List expansion in Lexer.AddTrivia, CreateWhitespaceTrivia func	https://github.com/dotnet/roslyn/blob/main/src/Compilers/CSharp/Portable/Parser/Lexer.cs#L2510

[NTaylorMullen]

@ryanbrandenburg moving this to the external tracking project bit