Improve File Block text handling and flexibility

[aurooba]

Description

Fix for #10967

I replaced the RichText component with a PlainText component as seemed to be decided in the referenced issue. However, I'm not sure this is the best fix. To make it look cleaner, I did remove the border and box shadow on the textarea element, but textarea naturally wants to have some space around it, which means it's not as accurate a representation of the front-end.

How has this been tested?

I tried out the File block, saved it, changed it, and tried pasting linked text in it (as reported in the issue). All seems to work just fine.

Screenshots

With Plain Text in the Editor:

Front end output:

Types of changes

Bug fix: Changes one of the editable elements so copy/pasting formatting doesn't come through.

Question

Does removing the border/box shadow around the input make this element less accessible?

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).
• I've updated related schemas if appropriate.

[mtias]

Also related #39118

[paaljoachim]

Hey @aurooba it would be great with a status update on this PR.
Thanks!