[Feature] Sync styles using native browser storage sync

[sagebind]

I'd like to sync my styles across machines, but would rather not use Google Drive for this purpose. Firefox and Chrome both now offer built-in mechanisms that allow extensions to synchronize arbitrary key-value pairs across multiple devices using the user's online account already signed in to their browser. This would not only be simpler and less error-prone than using the Google Drive API, it is also potentially a bit more secure and private.

Add an option to choose between multiple sync backends, allowing the existing Google Drive implementation as one option. Add a new backend using browser account storage. Note that these APIs do have some storage limits, so it is worth indicating that tradeoff when picking which backend.

• Firefox API: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/storage/sync
• Chrome & Edge API: https://developer.chrome.com/docs/extensions/reference/storage/#property-sync