Let "return" exit a script (#8148)

Currently, if a "return" is given outside of a function, we'd just
throw an error.

That always struck me as a bit weird, given that scripts can also
return a value.

So simply let "return" outside also exit the script, kinda like "exit"
does.

However, unlike "exit" it doesn't quit an interactive shell - it seems
weird to have "return" do that as well. It sets $status, so it can be
used to quickly set that, in case you want to test something.

tests/checks/return.fish

@@ -0,0 +1,22 @@
+#RUN: %fish -C 'set -l fish %fish' %s
+# Some tests of the "return" builtin.
+
+$fish -c 'return 5'
+echo $status
+# CHECK: 5
+
+$fish -c 'exit 5'
+echo $status
+# CHECK: 5
+
+$fish -c 'echo foo; return 2; echo bar'
+# CHECK: foo
+# but not bar
+echo $status
+# CHECK: 2
+
+$fish -ic 'echo interactive-foo; return 69; echo interactive-bar'
+# CHECK: interactive-foo
+# but not bar
+echo $status
+# CHECK: 69