Rename Provider/Notifier/AsyncNotifier

[rrousselGit]

In 3.0, some classes will likely be renamed:

Provider > SyncProvider
Notifier > SyncNotifier
AsyncNotifierProvider > FutureNotifierProvider
AsyncNotifier > FutureNotifier
ProviderBase > Provider
NotifierBase > Notifier

This removes the weird Base in the name of some abstract classes.

And it aligns FutureProvider and AsyncNotifier ; similarly to how we have StreamProvider and StreamNotifier.

[rrousselGit]

Provider>SyncProvider and the Notifier variant are not necessary btw.
If we also do #3904 then it's find to keep Provider as is.

But I think it's important to rename AsyncNotifier to FutureNotifier

[rrousselGit]

In case it wasn't obvious, we'd have a migration command. Possibly using dart fix itself if I manage to use it. Otherwise a custom one