TextArea theme improvements

[darrenburns]

Since the default behaviour of TextArea has changed to be more general purpose text editing instead of code editing, we can make the DX of themes nicer.

Yielding a plain TextArea should result in a text area which fits nicely with the default Textual theme system.

If you use a TextAreaTheme, and unfilled attributes on that theme will fallback to sensible Textual defaults. We'll add some component styles to cover these fallbacks too.

This PR will also include hiding the cursor when the TextArea loses focus.

Estimating around 1.5 days work for this, including testing.

[github-actions]

Don't forget to star the repository!

Follow @textualizeio for Textual updates.