Add a way to set outline for text

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

It is often useful to render text with an outline. Typical use case is for subtitle, it allows making the text readable independently of the background.

It is also useful in general for style.

Bevy doesn't expose any way to define a text outline, your best bet currently is to nest the text in a ui node with a semi-transparent background.

## What solution would you like?

I'd like to be able to define text with outlines, both for UI and `Text2d`.

## What alternative(s) have you considered?

Your best bet currently is to nest the text in a ui node with a semi-transparent background. It works, but may not fit the game's art style.