Implement FlxFlicker.pause() and FlxFlicker.resume()

[EliteMasterEric]

During the development of Funkin', I found the need to pause an active FlxFlicker without actively destroying it and recreating it. This PR adds that functionality.

[Geokureli]

Down to merge, but note that FlxTween.flicker now exists which has all the same tween methods for pausing, resuming and cancelling. It also allows for custom "ease" logic, so you can use one tween to do a flicker that changes frequency rather than using two calls, like you mentioned

[EliteMasterEric]

Down to merge, but note that FlxTween.flicker now exists which has all the same tween methods for pausing, resuming and cancelling. It also allows for custom "ease" logic, so you can use one tween to do a flicker that changes frequency rather than using two calls, like you mentioned

Good to know, thanks for that.

[Geokureli]

Thanks!