LibWeb: Add initial implementation of Element.blur()

This implementation includes a first cut at run the unfocusing steps
from the spec, with many things left unimplemented.

The viewport related spec steps in particular don't seem to map to
LibWeb concepts, which makes figuring out if things are properly focused
much more difficult.

Userland/Libraries/LibWeb/HTML/HTMLElement.h

@@ -44,6 +44,8 @@ public:
 
     void click();
 
+    void blur();
+
     bool fire_a_synthetic_pointer_event(FlyString const& type, DOM::Element& target, bool not_trusted);
 
     // https://html.spec.whatwg.org/multipage/forms.html#category-label