Add start of textmate language grammar.

Basic* grammar to start of with, this doesn't handle custom ops and doesn't
handle ops with regions. But useful enough to make reading the .mlir files
easier.

Followed the approach used for emacs & vim and placed in separate directory
under utils.

* I got a little bit carried away trying to handle attributes and tried to do some custom op printing handling, but finally abandoned it. Also first time writing a textmate grammar so I assume a lot can be improved :)

PiperOrigin-RevId: 262985490