Block API: Pasting content that results in a new block shouldn't be treated as inline content.

[pento]

Description

When pasting text, the paste handler currently assumes that if the text doesn't have a line break, and it results in one block, it can be treated as inline text, instead.

This may be correct for core, but is not necessarily correct for plugins that may wish to transform a single line of text into a custom block.

This PR tightens the paste handler assumption to only allow a selection of blocks to be treated as an inline paste.

Fixes #18390.

How has this been tested?

Tested with the sample plugin in #18390.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• 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.

[ellatrix]

Could some sort of test be added?

[pento]

The new supports.pasteTextInline property defaults to false, so blocks will need to opt-in to being overridden when pasting inline text.

I've added some tests, too.

[ellatrix]

@pento I marked pasteTextInline as an unstable API as I'm not expecting any non core blocks to use this feature (as you have noted yourself). Let's see if there are more cases popping up.

[ellatrix]

Thanks for the PR, this makes sense! :)