feat: Add a `panicdoc` macro

This would be useful for panicking with a multi-line message.

If you think this is a good idea, I can make the PR (it seems implementing this is pretty easy, because it works just like `printdoc`)

Start by locating the existing `printdoc` macro and its tests, since the issue says `panicdoc` should work the same way. Add the analogous macro for panicking with a multi-line message, then run the relevant test suite and verify the new macro's output and panic behavior.

Written by the indexing model from the issue text.