Support for before

[haejunejung]

Hello, @raon0211!
Can I work before of #91 ?

If it's okay to work on it, before and after seem very similar. Is it okay if I dug up a new issue for after and work on it together?

[raon0211]

Sure!

[haejunejung]

I've implemented the before function as below.

export const before = <P, F extends (...args: any[]) => P>(n: number, func: F): F => {
  if (!Number.isInteger(n) || n <= 1) {
    throw new Error(`n must be an integer greater than one.`);
  }

  let counter = 1;
  return ((...args: Parameters<F>) => {
    if (counter >= n) {
      throw new Error(`Function has been called ${n - 1} times already, which is the limit.`);
    }

    counter++;
    return func(...args);
  }) as F;
};

If it looks good, I proceed with enhancing documentation and then I submit a pull request!