AK: Make JSON parser return ErrorOr<JsonValue> (instead of Optional)

Also add slightly richer parse errors now that we can include a string
literal with returned errors.

This will allow us to use TRY() when working with JSON data.

Userland/Libraries/LibJS/Runtime/JSONObject.cpp

@@ -352,7 +352,7 @@ JS_DEFINE_NATIVE_FUNCTION(JSONObject::parse)
     auto reviver = vm.argument(1);
 
     auto json = JsonValue::from_string(string);
-    if (!json.has_value())
+    if (json.is_error())
         return vm.throw_completion<SyntaxError>(global_object, ErrorType::JsonMalformed);
     Value unfiltered = parse_json_value(global_object, json.value());
     if (reviver.is_function()) {