Rename pos to index on Node

[bitbrain]

Why this change?

When calling Node::move_child() it currently expects to_position which can get very confusing when using GDScript. I was wondering why I was unable to pass an actual position (Vector2) into it. Then I realized that by "position" we actually mean the "index" in the node hierarchy. To avoid mixing concepts together I decided to rename pos to index. This should not break compatibility as it is just an internal rename + documentation update.

[kleonc]

Kinda related to: #61418. But in here in case of Node::move_child its parameter seems to already be treated as an index, not as a position in a sense explained in #61418 (comment). Hence the renaming done seems correct.

The strange part to me is that child_count is treated as a valid index, it's treated equivalently to child_count - 1. Personally I'd make passing child_count be invalid since it's not a valid index. But oh well, it's already made like that (for whatever reason) so changing it would be breaking and thus it's for a potential separate PR (if changing it is desired at all).

@Calinou I don't think this PR breaks compatibility, it's only renaming the parameter.

[Mickeon]

The strange part to me is that child_count is treated as a valid index, it's treated equivalently to child_count - 1. Personally I'd make passing child_count be invalid since it's not a valid index. But oh well, it's already made like that (for whatever reason) so changing it would be breaking and thus it's for a potential separate PR (if changing it is desired at all).

I don't know. Compatibility can be kept by changing it to clamp instead of subtracting 1, which is less awkward, may I add.

[akien-mga]

Thanks!