Add SceneTree.get_node_count_in_group()

[alula]

Adds a more efficient way to get the number of nodes in specified group.

This extra method is an alternative to a common get_tree().get_nodes_in_group("group_name").size() use case, but without creating a redundant array.

Closes godotengine/godot-proposals#7080

[AThousandShips]

The documentation needs to be in alphabetical order, you can most easily accomplish this by using --doctool on the compiled editor version of your code, once that is fixed I or someone else can approve the workflows

Make sure to use git commit --amend when fixing this

[alula]

done!

[soloparatolos]

@AThousandShips Is there a possibility to get this reviewed and hopefully merged for 4.2.1? It would really helpful for the project I'm working on.

[AThousandShips]

Unlikely to be cherry picked I'd say, we mostly cherry pick bugfixes, it'd maybe get into 4.3, and we'd see about 4.2.x, 4.2.1 is a bug fix patch mainly so unlikely to get in that even if it's merged soon

[soloparatolos]

I build from master regularly so I'll keep an eye if it gets merged. Thanks and sorry to bother you.

[kleonc]

LGTM.

[YuriSizov]

Thanks!