Upgrade to the latest released version 0.8.0 of Bevy.

Add a new `bevy_asset` feature to enable animation of Bevy assets
(types implementing the `Asset` trait). Bevy 0.8 does not contain
`bevy_asset` in its defaul features, so this new feature reflects that
new possibility to build Bevy and í½ƒ Bevy Tweening without support
for the `bevy_asset` crate. The new feature is enabled by default for
discoverability and to prevent a behavior breaking change.

Allow querying the active tween of a sequence. Use this to improve the
`sequence` example with some text showing the current active tween and
its progress.

Import `bevy_tweening` from its old repository.

URL:
https://github.com/djeedai/bevy_extra/tree/main/crates/bevy_tweening

Commit: 1e13a72