Remove dependency on bevy_window from bevy_text

[alice-i-cecile]

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

This dependency is very tenuous: it only exists due to the reliance of Text2D on the window's scale factor.
It creates two small problems however:

1. It lengthens compile times by forcing these to be run in sequence.
2. It won't work well in a headless environment, or with an alternative windowing backend.

Options

1. Remove Text2D completely, by eliminating special-casing of world-space UI (World-space UI #5476).
2. Put all of Text2D behind an on-by-default feature flag.
3. Only put the "try and get the window's scale factor" code behind said feature flag.
4. Somehow communicate that information to bevy_text without a dependency.

Ultimately my preference is for 1: if we can manage that it will be a huge simplification.

3 seems strictly better than 2. 4 would be interesting, but I genuinely don't see how to do it unless we create a resource upstream of both crates that they can each read/write to.

Additional context

This is a write-up of a request by AbleHeathen on Discord.

[tim-blackbird]

It's important to note that bevy_window is also an indirect dependency of bevy_text through bevy_render.