by Anthony Fu @antfu
Implement a generic First<T> that takes an Array T and returns its first element's type.
For example:
type arr1 = ['a', 'b', 'c']
type arr2 = [3, 2, 1]
type head1 = First<arr1> // expected to be 'a'
type head2 = First<arr2> // expected to be 3
