feat(json): pretty print

[fernandolguevara]

Add json pretty print

User user = { .username = "admin", .created_at = 611667900 };
String json = json::marshal(tmem, user, { .chars = "\t" });
assert(json == "{\n\t\"username\": \"admin\",\n\t\"created_at\": 611667900\n}");

[lerno]

Better do this as an enum:

1. No pretty print
2. Tab
3. 1 space
4. 2 spaces
5. 3 spaces
6. 4 spaces
7. 5 spaces

With no pretty print, don't add \n either.

[lerno]

Thank you!