`Position` in `bevy_ui` has an overly general name

[Jondolf]

What problem does this solve or what need does it fill?

#18139 added a Position type to bevy_ui for a position relative to a UI node. This name is overly general and can easily conflict with similarly named types in user code or third party crates (such as Avian 0.3), especially because the type is in the prelude.

What solution would you like?

Rename the type to something more specific and UI-centric. Some ideas include UiPosition, or even GradientPosition, since that's what the type seems to be actually used for (unless future features also intend to use it).

[ickshonpe]

Yeah I almost called it UiPosition, I'd be okay with that. There were other names I considered like AnchoredPosition, ResponsivePosition, RelativePosition, etc but they all seemed a bit long.

There are other features that would need a type like this, so GradientPosition wouldn't work.