`flatten` accepts null/undefined type

[jonathanmhcai]

The signature flatten<T, R>(target: T, ...): R leaves T unconstrained, so TypeScript lets you pass null | undefined. However, at runtime the function expects an object and crashes.

Reproduction: https://playcode.io/2592924

Ideally this is prevented at compile time, by constraining T:

export function flatten<T extends object, R = Record<string, unknown>>(
  target: T,
  options?: FlattenOptions
): R;

Related: #185