Add support and documentation for data-only (silent) push notifications

[GavynHolt]

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Hello!

We at RSPNDR are dealing with Data-Only push notifications lately and have been unfortunately blocked by Skip's missing support for this. When the "notification" property (for example for the title and body of a push notification) is missing, the onMessageReceived function performs an early return which ignores the notification.

For us on the iOS side, this is working perfectly fine but requires an additional application delegate function to be implemented, so I've added some details in the documentation regarding this, including some notes about the possible requirement to disable swizzling.

On the Android side, I have implemented an additional custom "Messaging" didReceiveRemoteMessage function which is only triggered ever in the Android app. It didn't seem at all possible to make use of the UserNotificationCenter, so I hope this is an acceptable side step.

I have tested thoroughly that background notifications are indeed working on both iOS and Android, and the original banner-style push notifications are still working as originally designed.

Closes #86

[GavynHolt]

I should add that I did use Claude Fable to discuss some design ideas and changes and to assist with debugging, but the code is strongly vetted and tested by me personally, and only a very small portion of it was actually generated at all.

[marcprux]

Nice addition, and good docs, thanks!