LibHTML: Implement the <hr> element

This also meant I had to implement basic support for the border-styles
"inset" and "outset". If it's neither of those, we default to "solid".

Libraries/LibHTML/DOM/HTMLHRElement.h

@@ -0,0 +1,9 @@
+#pragma once
+
+#include <LibHTML/DOM/HTMLElement.h>
+
+class HTMLHRElement : public HTMLElement {
+public:
+    HTMLHRElement(Document&, const String& tag_name);
+    virtual ~HTMLHRElement() override;
+};