Feature-gate accessibility in bevy_ui

[Niashi24]

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

Due to bevy_ui's dependency on accesskit (and bevy_a11y) which depends on std, bevy_ui cannot currently support no_std. However, UI is foundational to many games, and other features of bevy_ui that would not depend on std (especially layouting) would be great to have in no_std. While accessibility features may be valuable to have in no_std, we are currently blocked on the ecosystem. In the meantime, these features should be feature-gated out of bevy_ui to allow for future no_std support.

What solution would you like?

Add an accessibility feature and gate integration with accesskit and bevy_a11y behind it. This should be enabled by default.

What alternative(s) have you considered?

Break out the base UI Node and layouting systems into a separate crate (bevy_layout?) that could then be made no_std on its own. This could be confusing for people but also might be worth it if bevy_ui gets too big in the future.

Additional context

no_std support for bevy_ui is also currently blocked on no_std support for bevy_ecs and bevy_app (#15460), and a dependency with bevy_render (#16313). These can be done in parallel however.

[DataTriny]

FYI the core of AccessKit, that is the accesskit crate, is no-std compatible now. Since the other adapter crates are meant to be compiled only on the relevant platforms, bevy_ui can keep AccessKit support and still be no-std compatible without a feature.