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/TreeBuilder.h

@@ -51,6 +51,8 @@ private:
 
     JS::GCPtr<Layout::Node> m_layout_root;
     Vector<JS::NonnullGCPtr<Layout::NodeWithStyle>> m_ancestor_stack;
+
+    u32 m_quote_nesting_level { 0 };
 };
 
 }