[mlir:LSP] Add support for keyword code completions

This commit adds code completion results to the MLIR LSP when
parsing keywords. Keyword support is currently limited to the
case where the expected keyword is provided, but a followup will
work on expanding the set of keyword cases we handle (e.g. to
allow capturing attribute/type mnemonics).

Differential Revision: https://reviews.llvm.org/D129184