LibWeb: Implement ImageStyleValue parsing

Later we will want to make a distinction between URL and Image values,
but this works for now.

Userland/Libraries/LibWeb/CSS/Parser/Token.h

@@ -76,6 +76,12 @@ public:
         return m_value.string_view();
     }
 
+    StringView url() const
+    {
+        VERIFY(m_type == Type::Url);
+        return m_value.string_view();
+    }
+
     bool is(NumberType number_type) const { return is(Token::Type::Number) && m_number_type == number_type; }
 
     int integer() const