Summary:
[Changelog][Internal]
This adds a method, `emitDeviceEvent` to the C++ API of TurboModules, which allows to make calls to JS's `RCTDeviceEventEmitter.emit` from a C++ TurboModules.
This is a very common pattern, specifically for the VR apps, but not only for them - e.g. Desktop fork also has a [custom implementation for this](https://www.internalfb.com/code/fbsource/third-party/microsoft-fork-of-react-native/react-native-utils/RCTEventEmitter.cpp).
Note that my original intent was to actually backport the latter, however there are some complications with wiring things in a robust way, without exposing too much stuff and relying on singletons or folly::dynamic.
So I ended up adding it to the TurboModule API itself and use the scheduler/JSI facilities instead.
This approach is arguably well self-contained, uses high level APIs, and shouldn't be abusable much.
Since I was trying to avoid usage of folly::dynamic in this case, I used a kind of "value factory" pattern instead in order to send the arguments to the JS thread in a thread safe way (see [the discussion here](https://fb.workplace.com/groups/rn.fabric/permalink/1398711453593610/)).
Reviewed By: christophpurrer
Differential Revision: D43466326
fbshipit-source-id: a3cb8359d08a46421559edd0f854772863cb5c39
407fb5c238