Added separate class for syncing multiple preference fields with stores

This will make the sync code less boilerplate-y. Instead of manually
subscribing back and forth, this will be done inside the class in
correct order and in any amount.

Types are a bit wonky though.

src/lib/extension/base/CacheablePreferences.ts

@@ -70,6 +70,10 @@ export class PreferenceField<
   set(value: Fields[Key]) {
     return this.#preferences.writeRaw(this.#fieldKey, value);
   }
+
+  get key(): Key {
+    return this.#fieldKey;
+  }
 }
 
 /**