fix: add int and string management on %json_add

[The-Lum]

Hello PlantUML team, and all,

To continue:

• feat: add new JSON builtin functions (%str2json, %json_add and %json_remove) #1742

And to fix:

• Dynamically create a json structure inside the preprecessor function #328 (comment)

Here is a fix for int and string, in order to manage:

%json_add({"age" : 30}, name, 123) '=> {"age":30,"name":123}

%json_add({"age" : 30}, name, Sally) '=> {"age":30,"name":"Sally"}

[FYI @philCryoport]
Reagrds,
Th.