MAINT: make typing module available at runtime

Closes https://github.com/numpy/numpy/issues/16550.

This makes `np.typing.ArrayLike` and `np.typing.DtypeLike` available
at runtime in addition to typing time. Some things to consider:

- `ArrayLike` uses protocols, which are only in the standard library
  in 3.8+, but are backported in `typing_extensions`. This
  conditionally imports `Protocol` and sets `_SupportsArray` to `Any`
  at runtime if the module is not available to prevent NumPy from
  having a hard dependency on `typing_extensions`. Since e.g. mypy
  already includes `typing_extensions` as a dependency, anybody
  actually doing type checking will have it set correctly.
- We are starting to hit the edges of "the fiction of the stubs". In
  particular, they could just cram everything into `__init__.pyi` and
  ignore the real structure of NumPy. But now that typing is available
  a runtime, we have to e.g. carefully import `ndarray` from `numpy`
  in the typing module and not from `..core.multiarray`, because
  otherwise mypy will think you are talking about a different
  ndarray. We will probably need to do some shuffling the stubs into
  more fitting locations to mitigate weirdness like this.

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