Also allow custom false and null when serializing to JSON

Also allow custom false and null when serializing to JSON

* doc/lispref/text.texi (Parsing JSON): Describe new arguments of
json-serialize and json-insert.

* src/json.c (enum json_object_type, struct json_configuration):
Move up in file before first usage.
(lisp_to_json_toplevel, lisp_to_json_toplevel_1, lisp_to_json):
Accept a struct json_configuration*.
(Fjson_serialize, Fjson_insert): Accept multiple args.
(json_parse_args): Accept new boolean configure_object_type.

* test/src/json-tests.el
(json-serialize, json-insert): Update forward decls.
(json-parse-with-custom-null-and-false-objects): Add assertions for
json-serialize.