ref.exist(provider) should be ref.read(provider.exists), to support ref.watch/listen

[rrousselGit]

A common ref.exist usage involves reacting to when the exist state changes. But there's no way to do that today besides calling ref.exist(provider) at regular interval.

We could instead make it a provider modifier, similar to provider.future:

bool exist = ref.watch(provider.exist);
if (exist) {
  return ref.watch(provider);
}