LibWeb: Track quote-nesting level while building the layout tree

This makes multiple levels of quote actually use different quotation
marks, instead of always the first available pair of them.

Each Layout::Node remembers what the quote-nesting level was before its
content was evaluated, so that we can re-use this number in
`apply_style()`. This is a bit hacky, since we end up converting the
`content` value into a string twice.

`StyleProperties::content()` now takes an initial quote-nesting level,
and returns the final level after that content.

Userland/Libraries/LibWeb/Layout/Node.h

@@ -175,6 +175,9 @@ public:
     SelectionState selection_state() const { return m_selection_state; }
     void set_selection_state(SelectionState state) { m_selection_state = state; }
 
+    u32 initial_quote_nesting_level() const { return m_initial_quote_nesting_level; }
+    void set_initial_quote_nesting_level(u32 value) { m_initial_quote_nesting_level = value; }
+
 protected:
     Node(DOM::Document&, DOM::Node*);
 
@@ -199,6 +202,8 @@ private:
     bool m_is_grid_item { false };
 
     GeneratedFor m_generated_for { GeneratedFor::NotGenerated };
+
+    u32 m_initial_quote_nesting_level { 0 };
 };
 
 class NodeWithStyle : public Node {