Update to Bevy 0.16

[benfrankel]

Notes:

• A version with Bevy 0.15 support was never published, so a release on this PR would jump straight from 0.14 to 0.16.
• bevy_egui has some new guidelines for library authors. In the interest of making this PR as self-contained as possible, I didn't update bevy_editor_pls to satisfy these guidelines.
• I made the straightforward translation from single() -> single().unwrap() and get_single() -> single() instead of trying to remove the panics.
• The new recommended naming convention for system sets is FooSystems, but I didn't make the change in this PR.
• The new Projection component means that queries can no longer fragment on the type of projection an entity uses, so now those entities will instead be skipped by a guard let-else if they don't use orthographic projection.

Testing:

• cargo run --example breakout compiled and ran, and pressing E toggled the editor UI.
• Seems to work in my personal game template.