Support plists in json.c

Support plists in json.c

* doc/lispref/text.texi (Parsing JSON): Mention plists and
json-serialize-use-plists.

* src/json.c (lisp_to_json_toplevel_1): Decide with
Vjson_serialize_use_plists.
(Fjson_serialize): Update docstring.
(enum json_object_type): Add json_object_plist.
(json_to_lisp): Can build plists.
(json_parse_object_type): Accept plists.
(Fjson_parse_string): Update docstring.
(json-serialize-use-plist): New DEFVAR_LISP.
(Qplist): New sym_of_json

* test/src/json-tests.el (json-serialize/object): Do some tests
with json-serialize-use-plists to t.
(json-parse-string/object): Parse something as a plist.