LibWeb: Don't generate ::before/::after for BR elements

We shouldn't be putting generated pseudo elements inside elements that
can't have children in the first place.

This patch fixes two issues:
- We stop generating pseudo elements for layout nodes that can't have
  children anyway.
- We mark Layout::BreakNode as not being able to have children.

Userland/Libraries/LibWeb/Layout/BreakNode.h

@@ -22,6 +22,7 @@ public:
 
 private:
     virtual bool is_break_node() const final { return true; }
+    virtual bool can_have_children() const override { return false; }
 };
 
 template<>