Implement a function getElementByTagName(root, tagName) that returns an array of all descendant elements of the given root element matching the specified tag name.

The function should:

• Traverse all descendants of the root element recursively.
• Match the tag name case-insensitively.
• Return an array of matching elements.

Example

const doc = new DOMParser().parseFromString(
  `<div>
      <span>One</span>
      <p>Two</p>
      <div><span>Three</span></div>
    </div>`,
  'text/html'
);

getElementByTagName(doc.body, 'span'); 
// Returns [<span>One</span>, <span>Three</span>]