ArraySlice / StringSlice: Fix behaviour with unions

[som-sm]

Fixes behaviour of ArraySlice with unions.

type Current = ArraySlice<[0, 1, 2, 3], 1, 2 | 3>;
//=> [1]

type Fixed = ArraySlice<[0, 1, 2, 3], 1, 2 | 3>;
//=> [1] | [2]

type Current1= ArraySlice<[0, 1, 2, 3], 2 | -3, 3>;
//=> []

type Updated = ArraySlice<[0, 1, 2, 3], 2 | -3, 3>;
//=> [2] | [1, 2]

This also fixes behaviour of StringSlice (Fixes #1289) with unions as it relies on ArraySlice, just needed the default type parameters to be fixed.

[som-sm]

This is complicated/verbose because the default value of Start/End is never, so we have to explicitly check for never before distributing, which introduces several branching cases.

I guess the reason for having the default value as never was that if End were simply Array_["length"], then union instantiations of End would give incorrect results.

For example,
ArraySlice<[0, 1, 2] | [0, 1], 0> would become
ArraySlice<[0, 1, 2] | [0, 1], 0, 3 | 2>,
which would then distribute to
ArraySlice<[0, 1, 2], 3 | 2> | ArraySlice<[0, 1], 3 | 2>,
and finally to
ArraySlice<[0, 1, 2], 3> | ArraySlice<[0, 1, 2], 2> | ArraySlice<[0, 1], 3> | ArraySlice<[0, 1], 2>,
which would be incorrect.

However, if we set the default for End to UnionMax<Array_["length"]>, that would fix this issue. The only thing is that explicitly passing never would no longer behave the same as omitting the argument. But that’s probably acceptable because never shouldn’t really be a valid value for Start/End anyway. In such cases, we can just return something like readonly unknown[] or [].

[som-sm]

However, if we set the default for End to UnionMax<Array_["length"]>, that would fix this issue. The only thing is that explicitly passing never would no longer behave the same as omitting the argument. But that’s probably acceptable because never shouldn’t really be a valid value for Start/End anyway. In such cases, we can just return something like readonly unknown[] or [].

@sindresorhus If this is fine, then I'll open a separate PR for this.

[sindresorhus]

Yes, seems fine.