Function onStart

onStart(cb, opts?): (() => void)
Register a callback in the engine start event.
This happens at the beginning of each frame
Parameters
- cb: LifecycleMethod
  The callback to be called
- Optionalopts: LifecycleMethodOptions
Returns (() => void)
A function that can be called to unregister the callback
- - (): void
  - Returns void
Example
```
onStart((ctx : Context) => {
    // do something
}
```