[KernelIntrisics] Define WorkItemNT type in package?

[christiangnrd]

Should we define this alias in the package?

Originally posted by @vchuravy in #746 (comment)

It's currently defined as a helper for the interface tests.

[christiangnrd]

@vchuravy I added to the milestone so we don't forget to get back to it in case we want to deal with this before KA 0.10