runtime(mail): add commentstring '> %s' to ftplugin

runtime(mail): add commentstring '> %s' to ftplugin

The new native commenting functionality is currently not used when
editing mail. One could reasonably expect it to change the "quote" state
of any given line in the mail (i.e. the preceding ">"), which would be
very handy and feel natural when editing mail. Especially since the
current file already uses "setlocal comments+=n:>".

Solution: Add commentstring to `> %s` to be used in files of type mail.

closes: #16669

Signed-off-by: Lucas Eekhof <105216949+eekhof@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>