Allow pasting a markdown block

What does this MR do and why?

When pasting a sole pre block containing markdown, it should be pasted as markdown, and not raw text. This fixes an issue where markdown copied from the docs isn't pasted correctly in the rich text editor.

Issue was brought up in #416293 (comment 1474565053). Related to #419106 (closed).

Screenshots or screen recordings

Copied content	Before	After
- Cat - Dog - Turtle	Screen Recording 2023-07-19 at 19.20.46.mov	Screen Recording 2023-07-19 at 19.22.12.mov

How to set up and validate locally

1. Go to Markdown docs.
2. Copy some markdown examples.
3. Paste in rich text editor.

MR acceptance checklist

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