To recap, this means `&` in the middle of a word no longer
backgrounds.
So:
```fish
echo foo&bar # prints foo&bar
echo foo& bar # backgrounds an echo that prints "foo" and runs "bar"
```
This is opt-in through a new feature flag "ampersand-nobg-in-token".
When this flag and "qmark-noglob" are enabled, this command no longer
needs quoting:
curl https://example.com/thing?foo=bar&duran=duran
Compared to the previous approach e1570a4 ("Let '&' only separate as
the first char of a word"), this has some advantages:
1. "&&" and "&>" are no longer affected. They are still special, even
if used between tokens without spaces, like "echo bar&>foo".
Maybe this is not really *better*, but it avoids risking to annoy
users by breaking the old variant.
2. "&" is still special if at the end of a token, like in "sleep 1&".
Word movement is not affected by the semantics change, so Alt-F and
friends still stop at every "&".
This correctly sets $status when a builtin succeeds but its output fails;
for example if the output is redirected to a file and that write fails.
Fixes#7857
This one tests a bunch of separate stuff, so we put it into a few
different files.
The main, new one is "slices.fish", which tests various index expressions.
