AK: Make JsonValue::as_string_or() const

Author: https://github.com/MaxWipfli
Commit: 13b2067da6
Pull-request: https://github.com/SerenityOS/serenity/pull/8293
Reviewed-by: https://github.com/alimpfard
Reviewed-by: https://github.com/awesomekling

AK/JsonValue.h

@@ -69,7 +69,7 @@ public:
     template<typename Builder>
     void serialize(Builder&) const;
 
-    String as_string_or(const String& alternative)
+    String as_string_or(String const& alternative) const
     {
         if (is_string())
             return as_string();