API: Align IVector with ES2021 proposal for 'at()'

[DLehenbauer]

Specifically, change 'getItem()' to just 'at()' and support negative index semantics:
https://github.com/tc39/proposal-relative-indexing-method

Excitingly, this means that Array, TypedArray, and String will just "happen to be" IVectorReaders, etc. the same way Map "happens to be" an IMapReader, etc.

EDIT: Proposal is now to add 'at()' rather than 'item()'