Bind constants related to 6DOF joints

[rburing]

This makes physics server GDExtensions able to use the respective enums without workarounds.

I didn't manage to document these, as they are unimplemented in Godot Physics. At this point I think it would be acceptable to bind them without documenting them, but of course suggestions for the documentation are welcome.

• Closes G6DOF_JOINT_FLAG_MAX (and a few others) is not defined in godot-4.0.2-stable tag godot-cpp#1104

cc @mihe

[mihe]

I'm happy to see them get added. This was on my to-do list somewhere at some point.

I can't speak to the documentation part of it, but the rest seems straight-forward and LGTM.

[akien-mga]

Thanks!