Array.unshiftの型バージョンを実装します。
例えば：
type Result = Unshift<[1, 2], 0> // [0, 1, 2]
import type { Equal, Expect } from '@type-challenges/utils'
type cases = [
Expect<Equal<Unshift<[], 1>, [1]>>,
Expect<Equal<Unshift<[1, 2], 0>, [0, 1, 2]>>,
Expect<Equal<Unshift<['1', 2, '3'], boolean>, [boolean, '1', 2, '3']>>,
]