Previously an environment variable to redefine would only be suggested if you
had not yet started typing one out. This makes it so that `env C<TAB>` will also
complete to, for example, [ `CC=`, `CXXFLAGS=`, ... ].
It also is smarter when suggesting variable names to complete: if a variable has
already been completed, it isn't suggested again. Additionally, it only suggests
names for variables that are exported, not all variables (the previous list was
insanely long and including things like all our `fish_...` variables).
This prevents a seemingly infinite loop in
fish -c 'alias ssh "env ssh"; complete -C "ssh "'
It still prints "maximum recursion depth exceeded", but a follow-up commit
will work around that.
Fixes#7389
It's now good enough to do so.
We don't allow grid-alignment:
```fish
complete -c foo -s b -l barnanana -a '(something)'
complete -c foo -s z -a '(something)'
```
becomes
```fish
complete -c foo -s b -l barnanana -a '(something)'
complete -c foo -s z -a '(something)'
```
It's just more trouble than it is worth.
The one part I'd change:
We align and/or'd parts of an if-condition with the in-block code:
```fish
if true
and false
dosomething
end
```
becomes
```fish
if true
and false
dosomething
end
```
but it's not used terribly much and if we ever fix it we can just
reindent.
This completed the commandline with options removed, which looked like
env VAR=VAL command option
Which didn't really actually work.
Fixes#5856.
[ci skip]
