Implement the advanced util type RequiredKeys<T>, which picks all the required keys into a union.
For example
type Result = RequiredKeys<{ foo: number; bar?: string }>;
// expected to be “foo”
import type { Equal, Expect } from '@type-challenges/utils'
type cases = [
Expect<Equal<RequiredKeys<{ a: number, b?: string }>, 'a'>>,
Expect<Equal<RequiredKeys<{ a: undefined, b?: undefined }>, 'a'>>,
Expect<Equal<RequiredKeys<{ a: undefined, b?: undefined, c: string, d: null }>, 'a' | 'c' | 'd'>>,
Expect<Equal<RequiredKeys<{}>, never>>,
]