Add batch insert function to LocalIndex

[JLambertazzo]

Batch insert would be very useful to us as we use Vectra to prototype RAG solutions and tend to load in a large amount of data at once.

Setup tests using node:assert to avoid modifying dependencies. Provided test cases pass

Also fixed a bug where the items array on _data was being passed by reference in the call to Object.assign. This was causing the third test case to fail because any mutation of this._update.items was also mutating this._data.items (see debug repl below).