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.cpp

@@ -46,6 +46,11 @@ CSSStyleDeclaration* CSSStyleRule::style()
     return m_declaration;
 }
 
+FlyString CSSStyleRule::qualified_layer_name() const
+{
+    return parent_layer_internal_qualified_name();
+}
+
 // https://www.w3.org/TR/cssom/#serialize-a-css-rule
 String CSSStyleRule::serialized() const
 {