Add keyframe markers into animation asset

[teakzc]

The animation_asset.luau file contains a markers table that collects keyframe markers, but other than that it is never used. So I propose that we return it in the animation asset table as such

return {
	keyframe_times = keyframe_times,
	keyframe_poses = keyframe_poses,
	length = length,
	keyframe_holes = keyframe_holes,
	keyframe_markers = marker
}