Implement a function getElementsByClassName(element, classNames) that finds descendant elements with the specified class names.

The function should:

• Accept an Element as the root.
• Accept a string of class names (separated by spaces) to match.
• Only search descendants of the element, not the element itself.
• Return an array of elements (not HTMLCollection).
• Not use querySelectorAll.

Example

const doc = new DOMParser().parseFromString(
  `<div class="foo bar baz">
      <span class="bar baz">Span</span>
      <p class="foo baz">Paragraph</p>
      <div class="foo bar"></div>
   </div>`,
  'text/html'
);

getElementsByClassName(doc.body, 'foo bar');
// [div.foo.bar.baz, div.foo.bar]