TypeScript类型体操

Typescript类型体操的总结和一些思考，可能有偏差或者有误解，后续会继续完善和总结，方便以后查阅。

Implement `If<C, T, F>` which accepts condition `C`, a truthy value `T`, a falsy value `F`.

Implement the built-in Exclude<T, U>，Exclude from `T` those types that are assignable to `U`

For given a tuple, you need create a generic Length, pick the length of the tuple

Implement a generic First<T> that takes an Array T and returns its first element's type.

Given an array, transform it into an object type and the key/value must be in the provided array.

Implement the built-in Readonly<T> generic without using it

Implement the built-in Pick<T, K> generic without using it.