Colorize langword inside doc comments

[DoctorKrolic]

Fixes: https://github.com/dotnet/roslyn/issues/63885

Looks like this:

[DoctorKrolic]

Still thinking about it though

I wonder, what arguments against do you have...

Here's a picture of the way I customized it:

Well, with this change you will get true colorized as a keyword, whatever color you have for it. That just makes sense: you have a keyword colorized as a keyword in the place, where it definitely stands for a keyword 🙂

[sharwell]

After reviewing the details, I'm fine with the design change as long as we can separate the new behavior out from ClassifyXmlTextTokens.