Add the ability to get timings of KeyframeMarkers

As a Roblox developer, it is currently impossible to find the times at which a keyframe marker occurs in an animation.
This is weird because we can find the times of named keyframes, but not the keyframe markers. I currently need this feature so that I can allow the server to fire events for animations without actually running the animation itself (which would cause a weird stutter effect for the user).

This could easily be solved by the addition of AnimationTrack:GetTimeOfKeyframeMarker(marker_name), which returns the times (in order) of the markers with the same name.

Also, it should be better specified in documentation what the difference is between a “Keyframe Name” and a “Keyframe Marker name”, as it is really confusing.