Move backends behind target-specific features

[fwcd]

This effectively depends on

• Platform target-specific features rust-lang/cargo#1197

...which would provide a clean way for users to choose their preferred backend, while minimizing friction for users that want nuit to "just work" with the standard backend for their platform (SwiftUI on macOS, GTK+ on Linux, ...).

Unfortunately, there isn't really a clean way to model this currently (April 2024), even in nightly Rust. Potential workarounds:

• Introduce wrapper crates for the individual backends that use the existing target-specific dependencies mechanism.
  • Drawback: Introduces a lot of boilerplate and we need to publish those crates to crates.io too, so we'd have to settle for some naming scheme
• Turn all backends into features, put the standard backend for every platform into the default features and make backends/features a no-op on unsupported platforms
  • This is effectively the approach taken by winit
  • Drawback: Potentially confusing, since enabling a backend feature would not necessarily imply building that backend (on unsupported platforms)

[fwcd]

We use the no-op approach for now.