Add ArrayLike an Dtype definitions to pandas._typing
The existing type hints in the codebase suggest a couple of easy additions to the pandas._typing module, including 

- [ ] ArrayLike = `Union[ABCExtensionArray, np.ndarray]` and
- [ ] Dtype = `Union[str, np.dtype, ExtensionDtype, Type]`.

These should be added to the typing module and the matching types replaced with the newly defined ones where they occur.

Crossreferences: #25829 and #25791.