patch 9.1.0538: not possible to assign priority when defining a sign

patch 9.1.0538: not possible to assign priority when defining a sign

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: #8334
closes: #15124

Signed-off-by: LemonBoy <thatlemon@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>