improve `ndarray.__getitem__` with rank-typing

[jorenham]

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 😅

[Kroppeb]

_[slice | array-indexer] returns Self for 1d+, otherwise rejects

This is wrong when the dimension is a Literal. I think it should be array[DTypeT, (int, *ShapeT[1:])]

[jorenham]

This is wrong when the dimension is a Literal

You're absolutely right.
But literal dimensions aren't supported. See #578 for why that is.