kernel: Add k_delayed_work_pending to check if work has been submitted

Add k_delayed_work_pending similar to k_work_pending to check if the
delayed work item has been submitted but not yet completed.
This would compliment the API since using k_work_pending or
k_delayed_work_remaining_get is not enough to check this condition.
This is because the timeout could have run out, but the timeout handler
not yet processed and put the work into the workqueue.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>