isWhitespaceNode(node) {
      return node.nodeType === 3 && node.textContent.trim() === "";
    }
Determines if a given DOM node is a whitespace text node.

A whitespace text node is a text node (nodeType === 3)
whose content consists only of whitespace characters (spaces, tabs, newlines).

@param {Node} node - The DOM node to check.
@returns {boolean} `true` if the node is a whitespace text node; otherwise `false`.
@private