Feature request: Offline queued method count and sync progress count

[wildhart]

Thanks for making isSyncing() available, and reactive/trackable!

I would like to diplay to my offline user:

• When offline: how many methods are in the sync queue.
• While syncing: how many methods have been synced / percentage sync progress.

I propose a new reactive/trackable method offlineStatus() which returns:

{
    queueLength: number,
    isSyncing: boolean, // this is a copy of the isSycing() method, just so we don't need to use both?
    syncCount: number,
    initialSyncCount: number,  
}

The purpose of the initialSyncCount number is so we can diplay the sync status as Syncing... 3 of 21, or Syncing... 10%. As the sync proceeds the queueLength would decrease, but the initialSyncCount would stay the same - just to make it easier to diplay the progress without having to remember the initial queue length. Looking at your code I think this would be easy to provide.

I'm happy to work on a PR if you want? If so, feel free to suggest alternative method name & response format, since this will be your API to maintain!