LibWeb: Implement HTMLElement.click()

This doesn't send the correct type of click event, but it does send
something, so it's already somewhat useful. :^)

Userland/Libraries/LibWeb/HTML/HTMLElement.idl

@@ -7,6 +7,8 @@ interface HTMLElement : Element {
 
     attribute DOMString contentEditable;
 
+    undefined click();
+
     // FIXME: Support the optional FocusOptions parameter.
     undefined focus();