Refactor parsing engine

[msujew]

We are currently looking into refactoring the parser+lexer infrastructure of Langium to support more parsers than just Chevrotain. Specifically, we want to introduce an abstraction layer for parsers and lexers (more than what we currently offer, which is very much tied to Chevrotain) and offer an antlr4 parser out of the box.

This issue is just to track progress on our work.