Move entitygroupfield into Group core

This would be up to @kristianvandeneynde, the maintainer of the Group module, primarily. I agree that it is a critical piece to every Group-based site I've ever built, so not sure what the likelihood is but a +1 from me, for sure.

Indeed. I'd be thrilled if this moved into group core, but that's totally upto @kristiaanvandeneynde to decide.

However, I'm sure he's wouldn't want to until 📌 Add compatibility with Group v2 and v3 Fixed and other fixes are complete. 😅 So moving this to postponed for now.

See also, #3152724: Improve architecture per kristiaanvandeneynde →

That said, 2.0.0-alpha1 is now out, which is compatible with Group V2 or V3 and D9 or D10. So this is more viable to consider again.