feat: is synchronized with form state

[Faust587]

Provide a way to find out if synchronization with localstorage is complete.

Issue

On first launch this triggers two renders, in my case it’s make two requests to the API.

Solution

After synchronizing the localstorage data and the form state, update the value of the isSynchronized state to true, so I can check it in my code and do the request.

[psugihara]

i had just copied the source to do the same edit locally, though I like your naming better than my checkedStorage :)