Lark Grammar for M2 language

This is related to issue #2889.

It would be convenient to have a formal model for the M2 language. This would in particular allow syntax checks of all files in the common library and packages.

Here is the Lark documentation:

https://lark-parser.readthedocs.io/en/stable/

A basic tentative was started in #2893, stuck at a preliminary stage.

This is remotely related to the question of highliting code, as allowed by

https://github.com/mahrud/language-macaulay2