Forbid subcommand keywords in variables-as-commands (#10249)

This stops you from doing e.g.

```fish
set pager command less
echo foo | $pager
```

Currently, it would run the command *builtin*, which can only do
`--search` and similar, and would most likely end up printing its own
help.

That means it very very likely won't work, and the code is misguided -
it is trying to defeat function resolution in a way that won't do what
the author wants it to.

The alternative would be to make the command *builtin* execute the
command, *but*

1. That would require rearchitecting and rewriting a bunch of it and
the parser
2. It would be a large footgun, in that `set EDITOR command foo` will
only ever work inside fish, but $EDITOR is also used outside.

I don't want to add a feature that we would immediately have to discourage.

tests/checks/expansion.fish

@@ -334,3 +334,9 @@ printf '<%s>\n' ($fish -c 'echo "$abc["' 2>&1)
 #CHECK: <fish: Invalid index value>
 #CHECK: <echo "$abc[">
 #CHECK: <           ^>
+
+set -l pager command less
+echo foo | $pager
+#CHECKERR: checks/expansion.fish (line 339): The expanded command is a keyword.
+#CHECKERR: echo foo | $pager
+#CHECKERR:            ^~~~~^