JsonValue: Add as_string_or(String)

Return the contained string if the value *is* a string, otherwise it
returns the alternative string passed in the parameter.

AK/JsonValue.h

@@ -63,6 +63,13 @@ public:
     template<typename Builder>
     void serialize(Builder&) const;
 
+    String as_string_or(const String& alternative)
+    {
+        if (is_string())
+            return as_string();
+        return alternative;
+    }
+
     String to_string() const
     {
         if (is_string())