Enable opening replay files on non-Flash targets #3661

Open Replay button in https://github.com/HaxeFlixel/flixel/blob/dev/flixel/system/debug/VCR.hx is disabled on all non-Flash targets although replay loading itself isn't Flash specific

Currently, onOpen() in https://github.com/HaxeFlixel/flixel/blob/dev/flixel/system/frontEnds/VCRFrontEnd.hx uses openfl.net.FileReference to select and read .fgr files, that can open files on all targets, not only on Flash

Maybe it'd be better to remove #if flash from VCRFrontEnd, or at least make it #if sys, so that recording gameplay isn’t purely Flash-only feature. I think this would be very useful