Use lastElement instead of lastItem

[Thomasan1999]

There was a similar proposal to use lastElement as an alias which I agree is not a good idea, the function should have a single name which should be lastElement. ECMAScript Language Specification uses the term element instead of an item and it is generally more known as an element.

Source: https://www.ecma-international.org/ecma-262/5.1/#sec-11.1.4

[trotyl]

Relates to https://github.com/keithamus/proposal-array-last/issues/20