port grammar to Antlr

[ericprud]

This would broaden the tool base.

[hsolbrig]

See: https://github.com/hsolbrig/pyjsg/tree/master/grammar . Grammar isn't identical to PrudJSG, something we need to get resolved.

[cmungall]

I am a fan of https://lark-parser.readthedocs.io/en/latest/grammar.html