Batch Async utils

FlxTweens have tween.then(otherTween) which allow chaining, but timers do not have anything like this. Would be nice to have batch timers like the one created for FNF by cyn0x8 in https://github.com/FunkinCrew/Funkin/pull/2391.

Even more so it would be nice if there was an interface or protocol shared by FlxTimer, FlxTween or any other async event that allowed them to be laid out in a single sequence, I.E.

new FlxSequence
(
    [
        FlxTween.tween(obj, {x: targetX }, 2.5, obj.doThing),
        FlxTween.flicker(1.0),// no callback
        FlxTween.tween(obj, {x: targetX }, 2.5, obj.doThing),
        FlxTimer().wait(1.0, doThing2),
        FlxG.camera.shake(0.05, 0.5),
        obj.startAsyncProcess(1.0)
    ],
    onSequenceComplete
)

the above would run each process in sequence, and then call onSequenceComplete

Start by reading the existing FlxTween chaining behavior and the FlxTimer API, then compare the batch timer implementation in Funkin pull request 2391. Define how a shared sequence would represent completion, callbacks, and async events such as tweens, timers, camera shake, and user processes. Done means a documented, consistent sequencing interface that supports the examples and invokes the completion callback at the end.