compareSpecificity = (a, b) => {
  for (let i = 0; i < 4; i += 1) {
    if (a[i] < b[i]) {
      return -1;
    } else if (a[i] > b[i]) {
      return 1;
    }
  }

  return 0;
}
Compares selector specificities.
Derived from https://github.com/keeganstreet/specificity/blob/8757133ddd2ed0163f120900047ff0f92760b536/specificity.js#L207

@param {import('./types.js').Specificity} a
@param {import('./types.js').Specificity} b
@returns {number}