preferences util: cut back on storageArea.set calls

set all unset preferences at once to avoid unnecessary repeated 
triggering of storage.onChanged

Author: AprilSylph

src/util/preferences.js

@@ -9,20 +9,25 @@
     const scriptManifest = await scriptManifestFile.json();
 
     const { preferences = {} } = scriptManifest;
+    const unsetPreferences = {};
     const preferenceValues = {};
 
     for (const [key, preference] of Object.entries(preferences)) {
       const storageKey = `${scriptName}.preferences.${key}`;
       const { [storageKey]: savedPreference } = await browser.storage.local.get(storageKey);
 
       if (savedPreference === undefined) {
-        browser.storage.local.set({ [storageKey]: preference.default });
+        Object.assign(unsetPreferences, { [storageKey]: preference.default });
         preferenceValues[key] = preference.default;
       } else {
         preferenceValues[key] = savedPreference;
       }
     }
 
+    if (Object.keys(unsetPreferences).length !== 0) {
+      browser.storage.local.set(unsetPreferences);
+    }
+
     return preferenceValues;
   };