Provide a "freeform" CKeditor/text format block

Problem/Motivation

Similar to the existing Freeform block in Gutenberg, it'd be nice if we could make use of the existing CKEditor widget (or theoretically any other non-Gutenberg text formats) to work with content in a way that's not supported by the Gutenberg WYSIWYG.

For example, working with tables might be a lot easier in CKEditor than using the native table editor. And might come with specific styles and settings that are difficult to migrate over.

This makes adoption of Gutenberg as the default editor easier, whilst still keeping all the advantages of your alternative editors.

This also help avoid situations where there's complex content that would otherwise be rendered as raw HTML, but can be conditionally handled through the alternative text format.

And can be a way to support using the alternate text formats in content as per ✨ Add/manage non-reusable content blocks (Drupal blocks) Fixed without having to create a custom block for it (which aren't very revision-friendly).

Proposed resolution

Provide a modal popup similar to ✨ Drupal Content Blocks are difficult to work with in the sidebar Active , where the user can use alternative non-Gutenberg text formats to alter the HTML.

Remaining tasks

Provide issue fork.

User interface changes

TBD.