Problem: not possible to assign priority when defining a sign
(Mathias Fußenegger)
Solution: Add the priority argument for the :sign-define ex command and
the sign_define() function (LemonBoy)
Use the specified value instead of the default one (SIGN_DEF_PRIO) when
no priority is explicitly specified in sign_place or :sign place.
fixes: vim/vim#8334closes: vim/vim#15124b975ddfdf9
Co-authored-by: LemonBoy <thatlemon@gmail.com>