Add Crystal language support

[spindlebink]

The built-in Ruby syntax works fine for Crystal files (I added a .cr file type to languages.toml) but there's already a Crystal tree-sitter on GitHub that I don't imagine would be too hard to add, providing somebody can write query files for it.

[spindlebink]

Alright, have taken a shot at writing the query for this, since Ruby's parser borks at the floored division operator //, and at a second look it seems that the linked implementation might not be complete. I know next to nothing about tree sitter implementations, though, so I could be mistaken--will probably need someone with expertise to check or possibly the author of the original repo @keidax to chime in.

[keidax]

That's correct, tree-sitter-crystal is very much a work in progress. I'll add a README notice to make that clearer.