Function resolveFormElement

Given any element matched by a CSS selector (which may be a shadow host, a wrapper <div>, or the form element itself), drill down and return the first reachable <input>, <select>, or <textarea>.

Resolution order:

1. The element itself, if it is already a form element.
2. The first form element inside the element's shadow root.
3. The first form element among the element's light-DOM descendants (including one level of shadow root on each descendant).