[mlir] Add basic tree-sitter grammar file

tree-sitter grammar file that tries to closely matches LangRef (it could use
some tweaking and cleanup, but kept fairly basic). Also updated LangRef in
places where found some issues while doing the nearly direct transcription.

This only adds a grammar file, not all the other parts (npm etc) that
accompanies it. Those I'll propose for separate repo like we do for vscode
extension.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D124352