LibHTML+Browser: Support scrolling to anchor with <a href="#foo">

This patch implements basic support for <a href="#foo"> fragment links.

To figure out where we actually want to scroll to, we have to do
something different based on the layout node's box type. So if it's a
regular LayoutBox we can just use the LayoutBox::position().

However, if it's an inline layout node, we use the position of the
first line box fragment in the containing block contributed by this
layout node or one of its descendants.

Libraries/LibHTML/HtmlView.h

@@ -23,6 +23,7 @@ public:
 
     void reload();
     void load(const URL&);
+    void scroll_to_anchor(const StringView&);
 
     URL url() const;