This means cleaning out old universal variables is now just:
```fish
abbr --erase (abbr --list)
```
which makes upgrading much easier.
Note that this erases the currently defined variable and/or any
universal. It doesn't stop at the former because that makes it *easy*
to remove the universals (no running `abbr --erase` twice), and it
doesn't care about globals because, well, they would be gone on
restart anyway.
Fixes#9468.
This now means `abbr --add` has two modes:
```fish
abbr --add name --function foo --regex regex
```
```fish
abbr --add name --regex regex replacement
```
This is because `--function` was seen to be confusing as a boolean flag.
This would print
```
abbr -a -- dotdot --regex ^\\.\\.+\$ --function multicd
```
which expands "dotdot" to "--regex ^\\.\\.+\$...".
Instead, we move the name to right before the replacement, and move
the `--` before that:
```
abbr -a --regex ^\\.\\.+\$ --function -- dotdot multicd
```
It might be possible to improve that, but this at least round-trips.
This renames abbreviation triggers from `--trigger-on entry` and
`--trigger-on exec` to `--on-space` and `--on-enter`. These names are less
precise, as abbreviations trigger on any character that terminates a word
or any key binding that triggers exec, but they're also more human friendly
and that's a better tradeoff.
This adds support for the `--function` option of abbreviations, so that the
expansion of an abbreviation may be generated dynamically via a fish
function.
Prior to this change, abbreviations were stored as fish variables, often
universal. However we intend to add additional features to abbreviations
which would be very awkward to shoe-horn into variables.
Re-implement abbreviations using a builtin, managing them internally.
Existing abbreviations stored in universal variables are still imported,
for compatibility. However new abbreviations will need to be added to a
function. A follow-up commit will add it.
Now that abbr is a built-in, remove the abbr function; but leave the
abbr.fish file so that stale files from past installs do not override
the abbr builtin.
This was supposed to act like `type -q` or `command -q`, in that it
returns 0 if at least 1 exists.
But because it used the wrong variable it didn't.
Fixes#8431.
