gh-76785: Minor Cleanup of "Cross-interpreter" Code

[ericsnowcurrently]

The primary objective here is to allow some later changes to be cleaner. Mostly this involves renaming things and moving a few things around.

• *CrossInterpreterData* -> *XIData*
• crossinterpdatafunc -> xidatafunc
• split out pycore_crossinterp_data_registry.h
• add _PyXIData_lookup_t

• Issue: [subinterpreters] PEP 554 implementation: add interpreters module #76785

[ZeroIntensity]

It looks like most of this is just find and replace renaming, but I can give it a once-over tomorrow if you want a second set of eyes.

[ZeroIntensity]

LGTM. There's not much to comment about here--it's all renaming as far as I can tell, and I don't think nitpicking formatting or something like that would be a good use of time :)