8. implement Extract<T, U>

As the opposite of Exclude<T, K>, Extract<T, U> returns a type by extracting union members from T that are assignable to U.

Please implement MyExtract<T, U> by yourself.

type Foo = 'a' | 'b' | 'c'

type A = MyExtract<Foo, 'a'> // 'a'
type B = MyExtract<Foo, 'a' | 'b'> // 'a' | 'b'
type C = MyExtract<Foo, 'b' | 'c' | 'd' | 'e'>  // 'b' | 'c'
type D = MyExtract<Foo, never>  // never