[enhancement] add timeline labels to ScrollTrigger markers

Hi GSAP Team,

It would incredibly helpful for finessing the timing of animations, and debugging timeline based animation issues if there was a method of visualising timeline labels with the 'markers' property - so that they were either automatically picked up when using them or could be passed in as an object of additional markers.

Even just being able to print static markers to the viewport would be super helpful - like in the SnorklTV ScrollTrigger course video that's referenced in the 'How does pinning work under the hood' section of the docs. Here they're using the markers to illustrate 100px increments in the dom to explain how the pin spacer works - with custom markers you could potentially pass in an object containing an array of labels and pixel values to track how animations line up to whatever you're aiming for with your sequence, without having to code up fake HTML elements whilst building out the animation.