feat(ios): emit tokenReceived event when FCM token is issued

[eljass]

What

Adds a tokenReceived event JS clients can subscribe to for live FCM registration token updates on iOS. Partially addresses #177 (Android follow-up planned).

Implementation:

• Plugin.swift messaging(_:didReceiveRegistrationToken:) now calls notifyListeners("tokenReceived", ["token": token])
• Gated on Messaging.messaging().apnsToken != nil so the pre-APNs registration token Firebase mints on first launch (which gets immediately replaced) is not emitted to consumers — this prevents persisting a stale token in the consumer's backend
• Deduped against lastNotifiedToken so foreground/background transitions do not re-emit the same token

JS API:

FCM.addListener('tokenReceived', ({ token }) => {
  await saveFcmToken(token);
});

Why

Issue #177 asks for a way to monitor token refresh without polling getToken(). iOS already had a MessagingDelegate callback wired up — it just discarded the token after caching. This change forwards the value to JS.

Test plan

• Listener fires once after PushNotifications.register() resolves (post-APNs)
• Does not fire for the pre-APNs token Firebase issues on first launch
• Does not fire twice for the same token across foreground/background transitions
• Verified in our app on iOS 14+ with Firebase iOS SDK 11.x

Notes

• getToken() behaviour is unchanged. Purely additive.
• No new dependencies.
• Belt-and-suspenders recommendation for consumers: still call getToken() once after PushNotifications.register() resolves, in case Firebase has already minted (and cached) a token before the listener attaches. Tokens minted before the listener registers are not buffered in this PR — adding a buffer would be a larger behavioural change and is left for a separate discussion.
• Android follow-up will arrive as a separate PR (uses @capacitor/push-notifications as a peer dependency, since FCM and PushNotifications cannot both register a FirebaseMessagingService independently). Issue How to Monitor token refresh on android/ios using this plugin #177 will close once both land.

Related PR

This stacks on top of #189 (fix(ios): refreshToken race), but the diffs do not overlap — both can be reviewed and merged independently.