Implement a type IsUnion, which takes an input type T and returns whether T resolves to a union type.
For example:
type case1 = IsUnion<string> // false
type case2 = IsUnion<string | number> // true
type case3 = IsUnion<[string | number]> // false
import type { Equal, Expect } from '@type-challenges/utils'
type cases = [
Expect<Equal<IsUnion<string>, false>>,
Expect<Equal<IsUnion<string | number>, true>>,
Expect<Equal<IsUnion<'a' | 'b' | 'c' | 'd'>, true>>,
Expect<Equal<IsUnion<undefined | null | void | ''>, true>>,
Expect<Equal<IsUnion<{ a: string } | { a: number }>, true>>,
Expect<Equal<IsUnion<{ a: string | number }>, false>>,
Expect<Equal<IsUnion<[string | number]>, false>>,
// Cases where T resolves to a non-union type.
Expect<Equal<IsUnion<string | never>, false>>,
Expect<Equal<IsUnion<string | unknown>, false>>,
Expect<Equal<IsUnion<string | any>, false>>,
Expect<Equal<IsUnion<string | 'a'>, false>>,
Expect<Equal<IsUnion<never>, false>>,
]