Allow custom "paste as"

[mbaas2]

I like the idea of the new "Paste As". Unfortunately it doesn't seem to cover my most common use case for it (or maybe I misunderstood the doc): when pasting a file[name], I'd like options to

• make absolute filename[s] relative to the edited file's location
• replace \ with /

And related: when pasting a URL into .md, wouldn't it be nice if VC checked if we're pasting into () and if not, would provide an option to do (perhaps even asking for the title of the link)?

[mjbvz]

Duplicate of #30066