实现 Flat，把数组类型扁平化

[zhengyimeng]

type Flat<T extends any[]> = T extends [infer First, ...infer Rest]
  ? First extends any[]
    ? [...Flat<First>, ...Flat<Rest>]
    : [First, ...Flat<Rest>]
  : [];

type F0 = Flat<[]>; // []
type F1 = Flat<["a", "b", "c"]>; // ["a", "b", "c"]
type F2 = Flat<["a", ["b", "c"], ["d", ["e", ["f"]]]]>; // ["a", "b", "c", "d", "e", "f"]