Provide text_format render element in JSON Form Widget

[dafeder]

Fixes #4132
Re-roll of #4133 by @stefan-korn

This provides an option in the UI Schema to define a Drupal text format for a textarea property. It only affects JSON Form widget forms, not any rendering or API response. We may do a follow-up to reflect this in the twig template or accompanying theme preprocess function. For now, what this achieves is replacing the bare textarea element on a form with a text_format element.

It does not allow a selection of all text formats available in Drupal, but locks a particular property in to a particular text format (as this is only about presentation and there is nowhere to store entity-specific format information).

I also added some test coverage to the mailto: fix as we kept getting dinged on that in coverage reports (even though that code needs to be replaced at some point).

QA Steps

• Add a basic HTML text format to Drupal (/admin/config/content/formats)
• Edit your dataset.ui.json so that the description property includes `"textFormat": "html"
• Add a new dataset. Confirm that you get the help text under the textarea that you would expect for the selected text format
• Now, enable the ckeditor module.
• Add another text format that uses ckeditor. Call it "ckeditor."
• Change the "textFormat" property in dataset.ui.json description to "ckeditor"
• Confirm that the dataset form now displays ckeditor for the description field

Checklist before requesting review

If any of these are left unchecked, please provide an explanation

• I have updated or added tests to cover my code
• I have updated or added documentation

[janette]

We held off as long as we could :)
Just the closing quote then this is good to go.