GetMiddleElement 
by 凤之兮原 @kongmingLatern
Get the middle element of the array by implementing a `GetMiddleElement` method, represented by an array
> If the length of the array is odd, return the middle element
> If the length of the array is even, return the middle two elements
For example
```ts
type simple1 = GetMiddleElement<[1, 2, 3, 4, 5]>, // expected to be [3]
type simple2 = GetMiddleElement<[1, 2, 3, 4, 5, 6]> // expected to be [3, 4]
```
