improve `ndarray.__getitem__` with rank-typing

It's a very complicated method signature, e.g.

• _[()] returns Self if 1d+, otherwise (if 0d) returns its scalar type
• _[int] returns Self[DTypeT, ShapeT[1:]] if 1d+, otherwise (if 0d) returns its scalar type
• _[int, int] returns Self[DTypeT, ShapeT[1:]] if 1d+, otherwise (if 0d) returns its scalar type
• ...
• _[None] returns array[DTypeT, (int, *ShapeT)]
• _[None, None] returns array[DTypeT, (int, int, *ShapeT)]
• ...
• _[slice | array-indexer] returns Self for 1d+, otherwise rejects
• _[slice | array-indexer, slice | array-indexer] returns Self for 2d+, otherwise rejects
• ...
• _[...] returns Self for 0d+

almost all possible combinations are also valid 😅