Rename remove() to remove_at() when removing by index

[AaronRecord]

Part of #16863
Closes godotengine/godot-proposals#2885

[AaronRecord]

Rebased. There's still a number of methods (e.g. https://github.com/godotengine/godot/blob/master/editor/editor_data.cpp#L538, remove.+?\(int (p_)?[_a-zA-Z0-9]+?in?de?x) that could have _at appended to them, but I've chosen to only focus on the array/vector methods to keep this PR's scope more reasonable. If desired though, I could rename those methods, or they could be done in a separate PR etc.

[AaronRecord]

(rebased) @aaronfranke

Also I wonder if Array.insert(...) should be renamed to insert_at(...) for consistency. I think that'd probably make sense, but it should be done in a separate PR.
Edit: Actually insert(...) is probably fine as is, as I think the name is mostly self-explanatory.

[aaronfranke]

I think this is a good idea.

[akien-mga]

This was approved in PR review meeting (sorry for the delay), it just needs a rebase (or redo if that's easier).

[AaronRecord]

rebased.

[akien-mga]

Thanks!