Put Markdown content into the clipboard

[rybak]

Many of the Git hostings support editing comments, issues, PR descriptions in Markdown syntax. It might be useful for the userscript to put content with type text/x-gfm into the clipboard.

• GitLab's text editor automagically detects pasted Markdown and chooses it over plain text content.
• Blocked by a bug in GitHub's text editor, which cannot handle text/x-gfm correctly: https://github.com/orgs/community/discussions/65235