[web] Custom Flutter initialization should survive hot restart #108403

The current flutter.js initialization flow does not support hot restart with custom initialization, because after the original promise is resolved, it cannot be resolved anymore.

The current workaround is to remove the didCreateEngineInitializer method once it's been used, but that reverts the initialization of flutter to "Auto" (and defaults). This won't play well with the next steps of embedding flutter in a custom element, or any other data that we might want to pass from JS to a flutter app.

flutter.js should support multiple hot restarts, keeping the custom initialization. To do so, a new onEngineInitialized API might need to be added to the flutter loader, so programmers can subscribe to Hot Restart events, rather than waiting for the resolution of a promise (that only happens the first time)