LibWeb: Preload resources hinted by <link rel="preload">

If a page is nice enough to give us some preload hints, we can tell
RequestServer to get started on downloading the resources right away,
instead of waiting until discovering them later on during parsing.

Userland/Libraries/LibWeb/HTML/HTMLLinkElement.h

@@ -32,9 +32,12 @@ private:
         enum {
             Alternate = 1 << 0,
             Stylesheet = 1 << 1,
+            Preload = 1 << 2,
         };
     };
 
+    RefPtr<Resource> m_preload_resource;
+
     CSSLoader m_css_loader;
     unsigned m_relationship { 0 };
 };