Remove experimental designation from bevy_ui_widgets feature flag

[alice-i-cecile]

Objective

bevy_ui_widgets is still incomplete and unstable, but the utility of the experimental` designation is questionable:

1. Running examples with required non-default features suck. This is frustrating for UI examples, and hampers adoption in our other examples even when real UI-based controls would be more elegant.
2. These widgets have proven useful to end users in practice, even in 0.19.
3. These widgets have seen a number of small bug fixes and improvements since their initial release.
4. It's not clear that this label is effective for slowing adoption, or if slowing adoption is currently desirable. Users don't have a ton of other great options: writing their own widgets will waste work and still need to be rewritten with BSN.
5. Bevy as a whole is still quite experimental. Unlike e.g. ghost nodes, there's no indication that we are considering wholly reverting these features.

Solution

Amended based on review feedback:

The experimental_ui_widgets feature have been renamed to ui_widgets.

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

Previous solution, from the original migration guide:

The experimental_ui_widgets and experimental_bevy_feathers features have been renamed to ui_widgets and bevy_feathers respectively.

The ui_widgets feature has been added to the ui feature collection (and thus bevy's default features) for ease of use.
The bevy_feathers feature remains off by default: it is primarily intended for use in dev tools, and should typically not be included in shipped end-user applications. As a result, it needs to be easy to enable and disable conditionally. This would be very challenging if it were a default feature or in a popular feature collection.

These crates remain immature, and subject to heavy breaking changes, even relative to Bevy's pre-1.0 standards.
However, they are useful enough to see wider adoption, and this changes substantially improves the user experience when setting up new projects and running Bevy examples.

[github-actions]

You added a new example but didn't add metadata for it. Please update the root Cargo.toml file.

[viridia]

The rationale for making feathers experimental is stronger than the one for making widgets experimental.

We know feathers is going to change because of bsn. There's no corresponding argument for bevy_ui_widgets.

[cart]

Agreed. I'm much more on board for removing the experimental designation for bevy_ui_widgets. I think feathers needs experimental solely for marketing reasons: the current user-facing interface is a bit too quirky, and it will be a large breaking change when BSN lands.

[github-actions]

You added a new example but didn't add metadata for it. Please update the root Cargo.toml file.