Grammar refactor

[d-jeffery]

When time allows, we can build off the following PR and improve the tokenization of the lexer and parser.

https://github.com/openfga/language/pull/22

Core problem stopping this is the character chosen to trigger a comment - it is very difficult to match with best practices in ANTLR4. https://github.com/antlr/grammars-v4/blob/3ecefc5f8ed0a38ff4b68705bcfd521bcd59f8d3/python/python2-js/Python2.g4#L454