Add public IThreadPoolWorkItem (#20387)

Add public IThreadPoolWorkItem (#20387)

- Changes the internal IThreadPoolWorkItem to be public, removing the legacy ThreadAbortException from it (which was specific to Task, anyway).
- Removes the IThreadPoolWorkItem implementation from Task, so that devs can't write code like `ThreadPool.UnsafeQueueUserWorkItem(task);` or `((IThreadPoolWorkItem)task).Execute();`, both of which could end up doing a variety of bad things that could show up in a variety of ways, some discoverable, some less so.
- Adds an internal UnsafeQueueUserWorkItemInternal that takes object so that it can be passed either an IThreadPoolUserWorkItem or a Task,
- Changes the ThreadPool's queues to be in terms of object instead of IThreadPoolWorkItem
- Changes the dispatch loop to type check for IThreadPoolWorkItem or Task so that both remain supported.