abbr: Warn when -U is given

[faho]

This prints a warning to stderr and then still does the thing.

Because of the error trailer, it points to the abbr help page.

TODOs:

• [N/A] Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.rst
• Wording? I tried to keep it reasonably general so it stands a chance of being reused, maybe something more specific is useful?
• This can now warn on every startup, if you have abbr -U in your config.fish. I don't believe there's anything better we can do (we don't want to store a universal variable or write a file to mark that we've errored)

[faho]

The big problem here is that it has no chance of warning in what might be the most common case - abbr without an explicit scope (which was an implicit universal), so I'm not sure this is worth it.

[ridiculousfish]

This can't hurt at least.