lint: Enforce consistent return

While all javascript functions have a return value - either an explicit
one from a return statement, or an implicit "undefined" - mixing both in
the same function is almost certainly an oversight, and more often than
not a bug.

Enable the corresponding eslint rule to catch those errors.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/635

lint/eslintrc-shell.json

@@ -7,6 +7,7 @@
                 "allow": ["^vfunc_", "^on_"]
             }
         ],
+        "consistent-return": "error",
         "key-spacing": [
             "error",
             {