`bevy_feathers` testbed example

## What problem does this solve or what need does it fill?

The `bevy_feathers` crate is quite complex, making it easy to introduce subtle bugs. It can be challenging to write conventional tests for visual theming issues.

## What solution would you like?

A dedicated feathers testbed example using a variety of themes and features to help verify that styling is applied correctly.

This could be included in `testbed_ui`, but IMO it's better to keep `testbed_ui` focused on lower-level technical concerns such as layout and rendering, and have a separate testbed for Feathers-specific theming and styling.