`experimental_ui_widgets` feature is no longer experimental

The experimental_bevy_ui_widgets feature has been renamed to bevy_ui_widgets.

The bevy_ui_widgets feature has been added to the ui feature collection (and thus bevy's default features) for ease of use.

This crate remains immature, and is subject to heavy breaking changes, even relative to Bevy's pre-1.0 standards. However, it is useful enough to see wider adoption, and this change substantially improves the user experience when setting up new projects and running Bevy examples.