bevy_reflect should be an optional feature in bevy_ui

_Originally discussed by @viridia in https://github.com/bevyengine/bevy/pull/20032#discussion_r2193048816_

The same thing should be done for bevy_feathers and bevy_core_widgets, probably in a single PR.

This is useful because reflection adds serious compile time and binary size, and isn't useful for all projects, especially for shipped apps.

Start with the discussion in PR #20032, then inspect the feature and dependency configuration for bevy_ui, bevy_feathers, and bevy_core_widgets. Confirm how reflection is currently enabled and identify the relevant crate checks or tests. Done means reflection can be disabled for these crates without breaking their existing functionality.

Written by the indexing model from the issue text.