ci: Switch ESLint rule eqeqeq to smart mode

This allows comparisons like != null and == null while still erroring
on other usages of == and !=.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2363>

lint/eslintrc-shell.yml

@@ -4,6 +4,9 @@ rules:
     - properties: never
       allow: [^vfunc_, ^on_]
   consistent-return: error
+  eqeqeq:
+    - error
+    - smart
   key-spacing:
     - error
     - mode: minimum