temp.sh: Add helper to create temp directory and rm -rf on EXIT

[nhooyr]

e.g. pick rn creates a new temp file on each invocation and never deletes.

I wrote a basic implementation but never tested how it works when the directory/file name moved already exists in $TMPDIR.
So it's not used anywhere.

4272a4f

[nhooyr]

Other than in tests to preserve diff artifacts. Or maybe on non zero exit code we don't delete, something like that.