[mlir] Significantly overhaul the textmate grammar

The current grammar is really crusty, only supports a handful of
cases, and is also out-of-date after various refactorings. This commit
refactors the textmate grammar to handle significantly more cases,
and now provides proper coloring for a majority of cases (including
dialect attributes, operations, types, etc.)

Differential Revision: https://reviews.llvm.org/D125458