LibWeb: Add Layout::Node::debug_description()

This returns a String with this format:

- LayoutNodeClassName<TAG_NAME>#id.class1.class2.class3

I've rewritten this function 10+ times at this point, so I'm just gonna
add it to the repository for future debugging needs. :^)

Userland/Libraries/LibWeb/Layout/Node.h

@@ -74,6 +74,7 @@ public:
     bool is_root_element() const;
 
     String class_name() const;
+    String debug_description() const;
 
     bool has_style() const { return m_has_style; }