appendChild(element, child) {
    if (Array.isArray(child)) {
      child.forEach((oneChild) => this.appendChild(element, oneChild));
    }
    else if (child instanceof HTMLElement || child instanceof Text) {
      element.appendChild(child);
    }
    else if (child) {
      element.appendChild(this.text(child.toString()));
    }

    return this;
  }
Append different types of children.
@param {HTMLElement} element - The element to append to.
@param {HTMLElement} child - The child element to append.
@returns {this} - The instance of the element.