lint: Enforce arrow notation

We replaced all Lang.bind() calls with arrow functions or the standardized
Function.prototype.bind(), at least for the former eslint has some options
to ensure that the old custom doesn't sneak back in.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/609

lint/eslintrc-shell.json

@@ -3,7 +3,8 @@
         "object-curly-spacing": [
             "error",
             "always"
-        ]
+        ],
+        "prefer-arrow-callback": "error"
     },
 
     "overrides": [