LibWeb/CSS: Implement cascade layers (aka @layer)

This is done quite simply for now, there are certainly optimizations
that can and should be made later.

With this we now pass:
- http://wpt.live/css/css-cascade/layer-basic.html
- http://wpt.live/css/css-cascade/layer-important.html
- http://wpt.live/css/css-cascade/layer-statement-copy-crash.html
- http://wpt.live/css/css-cascade/layer-stylesheet-sharing-important.html
- http://wpt.live/css/css-cascade/layer-stylesheet-sharing.html
- http://wpt.live/css/css-cascade/layer-vs-inline-style.html

Userland/Libraries/LibWeb/CSS/CSSStyleRule.h

@@ -33,6 +33,8 @@ public:
 
     CSSStyleDeclaration* style();
 
+    FlyString qualified_layer_name() const;
+
 private:
     CSSStyleRule(JS::Realm&, Vector<NonnullRefPtr<Selector>>&&, PropertyOwningCSSStyleDeclaration&);