Text Picking

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

Text should be pickable, with an eye to implementing a first party editable text field.

## What solution would you like?

A picking backend for `Text` and `Text2d` that gives the position in text of the pointer interaction.

## What alternative(s) have you considered?

Third party crates, like `bevy_cosmic_edit` and `bevy_simple_text_input`. Though both crates could benefit from having text picking built in.

## Additional context

I'm working on a backend in this draft PR, comments and improvements welcome :) #17775