Add global_basis property to Node3D

[mihe]

Resolves godotengine/godot-proposals#6229.

This adds a global_basis property to Node3D, which is similar to the global_position property in that it's just a shorthand for global_transform.basis.

Useful for things like intersect_ray, since physics queries are all done in global-space.

If nothing else it makes the global_* part of the Node3D API more consistent with the non-global one.

[aaronfranke]

Makes sense to me, and the code looks good.

[akien-mga]

Thanks!