Custom measure func example

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

The `Measure` trait isn't well documented, it's all a bit magical. Even a lot of devs contributing to Bevy UI aren't aware that it exists.

## What solution would you like?

An example that uses a measure func to automatically control the size of a Bevy UI widget.

If someone has a nice idea for an example and wants to give this a go but isn't clear about how measure funcs work, I'd be happy to help with the `Measure` implementation.