[vim] Add initial syntax definition for .mir files

This initial definition handles the yaml container and the embedding of
the inner IRs. As a stopgap, this reuses the LLVM IR syntax highlighting
for the MIR function bodies--even though it's not technically correct,
it produces decent highlighting for a first pass.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D95552