Attached is a better patch, which addresses a lot of different areas of C# fontification.

Overall I think these are pure improvements:

- fontification of generic types with namespace-prefixes

- function-call parameters

- variable use in string interpolation

- implicitly typed parameters in lambda-expressions

- index-expressions

- variables used in return-statements

- variables used in binary expressions

What might warrant some review is the use of the (:match) selector to identify local variables (as opposed to properties or types) for a more consistent fontification. This is the first time I'm using it, and I see there's some repetition in the implementation... Could this be improved somehow?

Do we have a "emacsy" way or convention to do this which I could have applied, or is it still a bit early wrt treesitter based modes?