is-undefined

### Type description + examples

copy of https://github.com/sindresorhus/type-fest/blob/main/source/is-null.d.ts

```typescript
export type IsUndefined<T> = [T] extends [undefined] ? true : false;
```

### Type source

_No response_

### Search existing types and issues first

- [x] I tried my best to look for it