Join 
by Pineapple @Pineapple0919
Implement the type version of Array.join, Join takes an Array T, string or number U and returns the Array T with U stitching up.
```ts
type Res = Join<["a", "p", "p", "l", "e"], "-">; // expected to be 'a-p-p-l-e'
type Res1 = Join<["Hello", "World"], " ">; // expected to be 'Hello World'
type Res2 = Join<["2", "2", "2"], 1>; // expected to be '21212'
type Res3 = Join<["o"], "u">; // expected to be 'o'
```
