LibHTML: Start adding support for <link rel="stylesheet">

This patch adds basic support for external stylesheets. It currently
only works with file:// URLs.

We do a synchronous full relayout after loading a stylesheet, which is
definitely on the aggressive side, but it gives us something to work
on improving. :^)

Libraries/LibHTML/DOM/HTMLLinkElement.h

@@ -0,0 +1,21 @@
+#pragma once
+
+#include <LibHTML/DOM/HTMLElement.h>
+
+class HTMLLinkElement final : public HTMLElement {
+public:
+    HTMLLinkElement(Document&, const String& tag_name);
+    virtual ~HTMLLinkElement() override;
+
+    virtual void inserted_into(Node&) override;
+
+    String rel() const { return attribute("rel"); }
+    String type() const { return attribute("type"); }
+    String href() const { return attribute("href"); }
+};
+
+template<>
+inline bool is<HTMLLinkElement>(const Node& node)
+{
+    return is<Element>(node) && to<Element>(node).tag_name().to_lowercase() == "link";
+}