Autolink selected text on link paste

[jodator]

📝 Provide a description of the new feature

We have the #4715 (auto-link feature) so this might duplicate in some sense.

While using Notion I've discovered one handy feature by accident:

1. Select a text
2. Paste link
3. The selected text get a link added instead of being replaced by the pasted link text.

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[scofalik]

Business context:

• If clipboard contains URL and text is selected inside the editor, after paste, we want to apply link on the selection instead of replacing the selection with the clipboard content.
• If the selection includes more than one element, then we want to keep the earlier behavior (selection content is removed, clipboard content is inserted).
• Images on links are nicely handled, so we can apply the same logic for images (pasting link over image adds link to the image).
• We will handle just text and images. Tables or other widgets will work as earlier.
• Clipboard must include link and only link.
• If formatted link was copied (e.g. a link that had bold, etc.) then we also apply the described behavior.
• If text selected in the editor already has a link, we still replace that link (in other words, it doesn't matter if selected text has a link, we still apply described behavior).

Technical notes:

• When it comes to checking whether clipboard data is an URL, we have RegExps in Link plugin so we can look there.
• Clipboard also "saves" plain text contents, in addition to HTML contents, so we don't need to strip formatting on our own (dataTransfer type -> text/plain).
• The improvement should most probably land in Link plugin as clipboard should not know anything about links or other features.
• This improvement should use LinkCommand and should be applied only if LinkCommand is enabled. This covers integration with multiple parts of the editor.

[scofalik]

Aaaand it appears that our friend @mlewand already prepared a plugin for this some time ago: https://github.com/mlewand/ckeditor5-paste-link