Support for flattenDeep

[ssi02014]

Implement flattenDeep based on the recently added flatten.

/**
 * Utility type for recursively unpacking nested array types to extract the type of the innermost element 
 * 
 * ExtractNestedArrayType<(number | (number | number[])[])[]> // number
 * ExtractNestedArrayType<(boolean | (string | number[])[])[]>; // string | number | boolean
 */
type ExtractNestedArrayType<T> = T extends readonly (infer U)[] ? ExtractNestedArrayType<U> : T;

function flattenDeep<T>(arr: readonly T[]): ExtractNestedArrayType<T>[] {
  return flatten(arr, Infinity) as ExtractNestedArrayType<T>[];
}

[ssi02014]

@raon0211 Can I proceed with the task? If you have any additional thoughts, I'd appreciate it. 🙏

[raon0211]

Sure! Looks really great.