Add an easy check to see if "timeline" exists

[robinhoodk17]

It would be really useful to check if a timeline exists before calling it with Dialogic.start("timeline")

Add a method that returns true or false depending on if the timeline exists and can be called, such as:
if Dialogic.exists("timeline"):
Dialogic.start("timeline")

Describe alternatives you've considered
Currently, you can get if the timeline exists with, for example, DialogicResourceUtil.get_timeline_resource("timeline") != null but an easy to use method would make this easier to work with. In addition, I imagine that this is something that a lot of people could use.