Add text input example using tui-textarea

[reubeno]

Includes these two changes:

• Adds a textarea example using tui-textarea, which conveniently allows pluggable input.
• Adds support for REVERSE modifier to both backends; this seems to be used by many input widgets to render cursors, and is needed for this example.

Please let me know if you'd prefer this be split up into multiple separate PRs. And of course, would be great to get any feedback! This was the first step for me to experiment with using ratzilla to run an interactive shell.

Screenshot:

[orhun]

Just tested it and this looks good to me overall!

Had some suggestions for improvements :)

[reubeno]

@orhun Thanks for the quick feedback and suggestions! I've accepted your suggestions, renamed the example (textarea -> text_area), and made it more consistent with other examples in using ratzilla::ratatui.

Don't hesitate to let me know if there are any other adjustments you'd like me to make.

[orhun]

Awesome stuff, thanks!