Add feature flag to turn off %self (#10262)

This is the last remnant of the old percent expansion.

It has the downsides of it, in that it is annoying to combine with
anything:

```fish
echo %self/foo
```

prints "%self/foo", not fish's pid.

We have introduced $fish_pid in 3.0, which is much easier to use -
just like a variable, because it is one.

If you need backwards-compatibility for < 3.0, you can use the
following shim:

```fish
set -q fish_pid
or set -g fish_pid %self
```

So we introduce a feature-flag called "remove-percent-self" to turn it
off.

"%self" will simply not be special, e.g. `echo %self` will print
"%self".

tests/checks/features-percent-self1.fish

@@ -0,0 +1,4 @@
+#RUN: %fish --features=remove-percent-self %s
+
+echo %self
+# CHECK: %self

tests/checks/features-percent-self2.fish

@@ -0,0 +1,2 @@
+#RUN: %fish --features    'remove-percent-self' -c 'status test-feature remove-percent-self; echo remove-percent-self: $status'
+# CHECK: remove-percent-self: 0

tests/checks/status.fish

@@ -57,6 +57,7 @@ status features
 #CHECK: qmark-noglob            on  3.0 ? no longer globs
 #CHECK: regex-easyesc           on  3.1 string replace -r needs fewer \'s
 #CHECK: ampersand-nobg-in-token on  3.4 & only backgrounds if followed by a separator
+#CHECK: remove-percent-self     off 3.8 %self is no longer expanded (use $fish_pid)
 status test-feature stderr-nocaret
 echo $status
 #CHECK: 0