feat: add new JSON builtin function (`%json_set`)

[The-Lum]

From this request:

• https://github.com/plantuml/plantuml/issues/328#issuecomment-2089276726

And to continue:

• 1742

• 1757

Here is a PR with this new builtin functions:

• %json_set

%json_set({"age" : 30, "name":"Bob"}, name, Sally) '-> {"age":30,"name":"Sally"}

[FYI @philCryoport] Regards, Th.