LibWeb: Implement (most of) HTMLElement.focus()

The main deviation from the spec is that we don't have a straightforward
representation of the spec's "focusable area" concept.

I've left a bunch of FIXME's around for our future selves. :^)

Userland/Libraries/LibWeb/HTML/HTMLElement.idl

@@ -7,6 +7,9 @@ interface HTMLElement : Element {
 
     attribute DOMString contentEditable;
 
+    // FIXME: Support the optional FocusOptions parameter.
+    undefined focus();
+
     [LegacyNullToEmptyString] attribute DOMString innerText;
 
     readonly attribute long offsetTop;