Add a GQL model and parser

[joshsh]

It is time for some basic GQL support (https://www.gqlstandards.org), analogous to the current OpenCypher support. The problem is that the OpenGQL ANTLR grammar is four times as large as the OpenCypher grammar, to say nothing of the actual GQL BNF. GPT-4-based automation plus human quality control will be necessary. Let's see how it goes.

Since the property graph components of Hydra are getting so large, I am thinking of separating them out into their own module, hydra-pg, similar to hydra-ext. The TinkerPop/Gremlin support would be moved there, as well.

[joshsh]

The model is done; the parser may not be. All sources merged into main (though the branch has been deleted to avoid conflicts).