superui v0.4.0 (bevy 0.20)

Changed

• Target Bevy 0.20; rebased the Flair fork onto bevy_flair 0.9.

Per superui's versioning policy, a minor bump tracks a Bevy version bump and
carries only the changes needed to support it — there are no new superui
features or behavior changes in this release. main now tracks the Bevy 0.20
(0.4.x) line; the Bevy 0.19 (0.3.x) line continues as a maintained branch.