feat: add new JSON builtin functions (`%str2json`, `%json_add` and `%json_remove`)

[The-Lum]

Hello PlantUML team, and all,

To answer to:

• QA-15436
• QA-17474
• QA-18418

• 328

• 1733

Here is a (first) PR with the new builtin functions:

• %str2json
• %json_add
• %json_remove

---

[!NOTE] Here is a first question about %json_remove on array:

%json_remove([1,2,3], 0) = [2,3]

But currently:

%json_remove([1,2,3], -1) = [1,2,3]

• What should we do when the index is negative? (nothing or from the end of the array... like below)

  %json_remove([1,2,3], -1) = [1,2]

---

Regards, Th.