Improve Schema checks

[Reinmar]

• In insertContent()'s _tryAutoparagraphing() – checkChild( [ ...position.getAncestors(), paragraph ], node ) would be more reliable.
• Introduce checkWrap() and use in BlockQuoteCommand.
• Introduce checkRename() and use in HeadingCommand, ListCommand, ParagraphCommand.
• Introduce checkMerge() and use in insertContent() – see Paragraph shouldn't be pasted into the list item #730 (comment)
• The entire deleteContent() needs to be reviewed (https://github.com/ckeditor/ckeditor5-engine/issues/365).
• And perhaps more...

[pomek]

Introduce checkMerge() and use in insertContent()

https://github.com/ckeditor/ckeditor5-engine/issues/1251

[jswiderski]

#5962 - perhaps this issue could be handled by one of checks.

[CKEditorBot]

There has been no activity on this issue for the past year. We've marked it as stale and will close it in 30 days. We understand it may be relevant, so if you're interested in the solution, leave a comment or reaction under this issue.

[CKEditorBot]

We've closed your issue due to inactivity over the last year. We understand that the issue may still be relevant. If so, feel free to open a new one (and link this issue to it).