Mark observer as nonisolated and unsafe in MainActor

[AZero13]

No description provided.

[cthielen]

I'm uncertain about the tradeoff of introducing the runtime cost of a closure to silence a false positive concurrency warning. (We know @MainActor closures are called via @MainActor post()s, so we're not actually sending or losing main actor as the warnings imply, but the compiler doesn't know that.)

Two other options come to mind:

1. Use unsafeBitCast to simply change the signature of the closure, e.g. let unsafeObserver = unsafeBitCast(observer, to: (@Sendable (Message) -> Void).self), then pass unsafeObserver instead of observer. I think this is safe because we're only modifying annotations with no runtime representation.
2. A cleaner solution might be for the internal registrar that _addObserver() ultimately works with to take an enum of the two possible closure types instead. That, though, has a little bit of a memory cost that could become substantial in extreme cases where NotificationCenter has 10,000+ observers, though I don't think that's the typical case.