Make define-command accept more verbose/understandable argument-descriptors

[BierLiebHaber]

A comment on discord mentioned that strings as argument-descriptors are sub optimal, to which I agree.
This merge-request overhauls the define-command macro so that it accepts both old-style string descriptors, as well as list argument-descriptors.

The following descriptors were added:
:universal, :universal-1, :universal-nil, :string, :number, :num, :buffer, :other-buffer, :file, :new-file and :region
with :universal-1 and :num being aliases of :universal and :number respectively.

I also added some documentation to the existing :splice descriptor and exposed the extremely helpful comment in the command-function as a doc-string.

[cxxxr]

I think it's very nice.

By the way, I would like to avoid preparing num, what do you think?
I think we'll want str, too, and there's no end to it.

[BierLiebHaber]

I agree. If people want to type less they can still use the string-descriptors.