Add proper event for syncing

[AbdelAbouhassane]

Context

The Sync method uses behind the scenes first detach to detach all associations and then will use attachNew for each new entry, which leads to a lot of events being dispatched.

What did I do

• Add syncing and synced events
• Pass the results of sync to the dispatched event
• Support both Laravel 8 and 9
• Update README

[fico7489]

we can add a new event, but all tests must pass