Implement validation for simple extensions

[jvanstraten]

Everything related to the semantics of simple extensions currently just throws warnings. Only the schema for the YAML files is already validated.

Note that the largest/most important bit of logic that's needed is a type expression parser. The rest should be comparatively straightforward.

[jvanstraten]

Just to update on this for posterity: as of validator 0.0.9 the type expression system proposed in substrait-io/substrait#352 is currently implemented in the validator, though it is not really used by anything yet (extension parsing is still missing for the most part) and has not been thoroughly tested. With that PR immediately derailing for procedural and personal reasons, I'm back to square one of not knowing how to correctly implement these things, and have ceased my involvement with the project a lot more abruptly than I was intending to. Hopefully my code will be useful later on nonetheless, if only for inspiration, but if not, it should just be removed.