Newer method(s) missing from TrackedArray type

[elwayman02]

The types for TrackedArray don't include the newer (but fully supported) method findLast()

I think that means we just need to add it here, but not sure?

It may be worth doing a general review of all the built-ins to see what other methods have achieved full browser support since the initial implementation and ensuring those methods are properly accounted for since the built-ins are intended to have 1:1 functionality with their native counterparts.