Flush RN task queue with invokeAsync

[tomduncalf]

What, How & Why?

This closes #3571, #4389

React Native has its own JS "microtask" queue, implemented in https://github.com/facebook/react-native/blob/main/Libraries/Core/Timers/JSTimers.js. Any asynchronous work, e.g. setTimeout or setImmediate, is added to this queue, and the queue is flushed whenever a message is sent from React Native's native core to JS: https://github.com/facebook/react-native/blob/main/Libraries/BatchedBridge/MessageQueue.js#L108.

However, our native to JS messages are not being passed via this abstraction - instead, we hook directly into the JS engine. This means that React Native is not aware that when Realm has done some async work, we might need to update the UI (and therefore flush the task queue), and this can result in Realm-related UI updates not showing until some action is taken which causes React Native to send a message from the core to JS (e.g. touching the screen), which flushes this task queue, resolving pending promises and updating the UI.

After some discussion and research (e.g. facebook/react-native#33006), it was determined that exposing and calling the invokeAsync method from the React Native CallInvoker was the most reliable way of triggering a flush of this queue without having to modify React Native internals to directly expose their flush method – the other approaach we tried (exposing an internal React Native JS function to flush the queue, see #4330) was not 100% reliable.

We pass a lambda which calls this method (debounced, so we don't call it multiple times if it has not yet been invoked asynchronously) down from our RN module initialization code into the JSC code, which can then call this method whenever we make call from C++ to JS.

This issue could not be reproduced on Android, and the current Android JSC "trampoline" makes it hard to work out how to call into invokeAsync in the same way. However, it should be possible to port this fix to the Hermes branch for Android as that code has been reworked.

☑️ ToDos

• 📝 Changelog entry
• 📝 Compatibility label is updated or copied from previous entry
• 🚦 Tests
• 📱 Check the React Native/other sample apps work if necessary
• 📝 Public documentation PR created or is not necessary
• 💥 Breaking label has been applied or is not necessary

If this PR adds or changes public API's:

• typescript definitions file is updated
• jsdoc files updated
• Chrome debug API is updated if API is available on React Native