Add caching (optional?)

[bjornharrtell]

Perhaps https://learn.microsoft.com/en-us/azure/devops/pipelines/tasks/reference/cache-v2?view=azure-pipelines can be used as is.. but I'm actually not sure how to do it, so it would be good if it was just part of this task.. or some FAQ entry on how it can be used with this task.

[bjornharrtell]

I note that the typical instruction is to use $(Pipeline.Workspace)/.yarn as the cache path but it doesn't seem to find that...