Redux generated-runtime native exit adapter

In Emscripten 6.0.13 the native cancellation import is an inline function
inside the import table. Its unique body is adapted as follows:

Before: MainLoop.pause();MainLoop.func=null
After:  MainLoop.pause();MainLoop.func=null;Module["onMainLoopExit"]?.()

The launcher returns to / after this explicit native shutdown. Pausing
does not emit the callback. Redux 0.4.5 uses a rebuilt C++ WASM module;
Redux 0.4.4 and 0.4.4.1 used the unchanged upstream prebuilt module.
