Add a source generator attribute for global groups.

[willnationsdev]

Seeing as how there are already attributes for layer names and the input map, it would also be helpful to have a similar one for global group names (in ProjectSettings > Global > Groups, listed in project.godot under the [global_group] section).

May take a crack at a PR for this since I could use your own code as a template for it.

[Cat-Lips]

Thanks Will. Good idea :)