LibWeb: Implement StyleSheet.type

This just returns "text/css" on CSSStyleSheet, nothing exciting.

Userland/Libraries/LibWeb/CSS/CSSStyleSheet.h

@@ -44,7 +44,9 @@ public:
         return adopt(*new CSSStyleSheet(move(rules)));
     }
 
-    virtual ~CSSStyleSheet();
+    virtual ~CSSStyleSheet() override;
+
+    virtual String type() const override { return "text/css"; }
 
     const NonnullRefPtrVector<CSSRule>& rules() const { return m_rules; }
     NonnullRefPtrVector<CSSRule>& rules() { return m_rules; }

Userland/Libraries/LibWeb/CSS/StyleSheet.h

@@ -41,6 +41,8 @@ public:
 
     virtual ~StyleSheet() = default;
 
+    virtual String type() const = 0;
+
     DOM::Element* owner_node() { return m_owner_node; }
     void set_owner_node(DOM::Element*);
 

Userland/Libraries/LibWeb/CSS/StyleSheet.idl

@@ -1,8 +1,8 @@
 interface StyleSheet {
 
     readonly attribute Element? ownerNode;
+    readonly attribute CSSOMString type;
 
-    // readonly attribute CSSOMString type;
     // readonly attribute USVString? href;
     // readonly attribute CSSStyleSheet? parentStyleSheet;
     // readonly attribute DOMString? title;