LibWeb: Remove is<BlockContainer> check for fragments in hit testing

This change introduces a method for direct access to the paintable of
a PaintableFragment. This method is intended to replace the usage of
the layout node pointer. Currently, we are only eliminating the use
of `layout_node()` in hit testing.

Additionally, we no longer check if a fragment's layout node is a
`BlockContainer` before recursing into its children during hit testing.
This check was likely relevant when all fragments were owned by
`PaintableWithLines`, but now it should be safe to remove this check.

Userland/Libraries/LibWeb/Painting/PaintableFragment.h

@@ -20,6 +20,7 @@ public:
     explicit PaintableFragment(Layout::LineBoxFragment const&);
 
     Layout::Node const& layout_node() const { return m_layout_node; }
+    Paintable const& paintable() const { return *m_layout_node->paintable(); }
 
     int start() const { return m_start; }
     int length() const { return m_length; }