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