Don't touch $SHLVL if not interactive

It's not super clear what $SHLVL is useful for, but the current
definition is essentially
"number of shells in the parent processes + 1"

which isn't *super useful*?

Bash's behavior here is a bit weird in that it increments $SHLVL
basically always, but since it auto-execs the last process it will
decrement it again, so in practice it's often not incremented.

E.g.

```
> echo $SHLVL
1
> bash -c 'echo $SHLVL; bash'
2
>> echo $SHLVL
2
```

Both bashes here end up having the same $SHLVL because this is
equivalent to `echo $SHLVL; exec bash`. Running `echo $SHLVL` and then
`bash -c 'echo $SHLVL'` in an interactive bash will have a different
result (1 and 2) because that doesn't *exec* the inner bash.

That's not something we want to get into, so what we do is increment
$SHLVL in every interactive fish. Non-interactive fish will simply
import the existing value.

That means if you had e.g. a bash that runs a fish script that ends up
opening a new fish session, you would have a $SHLVL of *2* - one for the
bash, and one for the inner fish.

We key this off is_interactive_session() (which can also be enabled
via `fish -i`) because it's easy and because `fish -i` is asking for
fish to be, in some form, "interactive".

That means most of the time $SHLVL will be "how many shells am I deep,
how often do I have to `exit`", except for when you specifically asked
for a fish to be "interactive". If that's a problem, we can rethink it.

Fixes #7864.

doc_src/language.rst

@@ -1248,7 +1248,7 @@ Fish also provides additional information through the values of certain environm
 
 - ``pipestatus``, a list of exit statuses of all processes that made up the last executed pipe.
 
-- ``SHLVL``, the level of nesting of shells.
+- ``SHLVL``, the level of nesting of shells. Fish increments this in interactive shells, otherwise it simply passes it along.
 
 - ``status``, the `exit status <#variables-status>`_ of the last foreground job to exit. If the job was terminated through a signal, the exit status will be 128 plus the signal number.