__fish_complete_suffix: complete all files, but sort files with matching suffix first

[krobelus]

Description

This is a compromise between the common case where the user only wants files with a matching suffix,
and the exceptional case when there is no file extension, or one file we don't know about.

See #7040 and others.

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.rst

[faho]

This looks like a great idea!

[krobelus]

Many times when completing one of these commands, a single Tab already selects the only sensible option.
It's a bit sad to lose that.
Still, completing non-matching files seems like the least contentious solution.