Implement first-in-class text editing widget

[PoignardAzur]

This is a group issue for all the things we want our text input code to handle.

First step will be to flesh out the list of things we want.

From the three-months roadmap:

We'd like to bring up Masonry's textbox widget to be best-in-class among Rust frameworks. We want to support native-feeling interactions out of the box, such as cursor movement (go to next word, go to beginning of line), selection (select to end of text) and edition (IME, deleting an entire word, etc). This is something other framework developers have expressed interest in, so we want to implement it in a composable way.