ENH: Add a typing protocol for representing nested sequences - #19894

>>> from __future__ import annotations
>>> from typing import TypeVar, TYPE_CHECKING
>>> import numpy.typing as npt

>>> T = TypeVar("T")

>>> seq_1d_a: list[int]
>>> seq_1d_b = [1]
>>> seq_2d_a: list[list[int]]
>>> seq_2d_b = [[1]]

>>> def func(array_like: npt._NestedSequence[T]) -> T: ...

>>> if TYPE_CHECKING:
...     # The good
...     reveal_type(func(seq_1d_a))  # Revealed type is "builtins.int*"
...
...     # The bad
...     reveal_type(func(seq_1d_b))  # Revealed type is "Any"
...     reveal_type(func(seq_2d_a))  # Argument 1 to "func" has incompatible type "List[List[int]]"; expected "_NestedSequence[<nothing>]"
...     reveal_type(func(seq_2d_b))  # Revealed type is "Any"