Show embedded language classification in syntax visualizer

[chucker]

Implements dotnet/roslyn#63664.

Working:

• Show ClassifiedSpans that are children of String tokens as tree nodes
• Show their properties
• Allow navigating from source to such a span
• Allow navigating from such a span back to source

Not working:

• Show the classification's color. I need help with this — Visual Studio always seems to return black as the color.