`ui` feature is now no longer implied by the `3d` or `2d` features

Swapping the UI framework for your Bevy project is a common form of customization. We think that users should be able to do this easily, without having to give up the ease of use (and updates!) that come with our top-level feature collections.

To achieve this, the ui feature collection is now no longer implied by the 3d or 2d feature collection.

To migrate:

• If you used all default features before, nothing changes for you.
• If you want to opt out of using bevy_ui, it is now as simple as disabling default features, and manually opting into 3d or 2d (and optionally audio).
• If you already opted into non-default features and want to continue using bevy_ui, you will now have to add the ui feature.