Bring Razor Theme definitions out of WebTools, into aspnetcore-tooling

[ryanbrandenburg]

Currently the definitions and color themes of Razor coloring classifications live in the WebTools repo, but since they'd really be better off in the aspnetcore-tooling repo. This would be the correct separation of concerns, reduce the number of cross-repo changes and WebTool insertions which are needed to test out colorization changes, and prevent us from randomizing the WebTools folks when we need to change colors.

I believe Roslyn has already done something similar with files like https://github.com/dotnet/roslyn/blob/main/src/VisualStudio/Core/Def/ColorSchemes/VisualStudio2019.xml, so we should look to their implementation for guidance.