Accept plists when serializing and parsing JSON

Accept plists when serializing and parsing JSON

* doc/lispref/text.texi (Parsing JSON): Mention plist support.

* src/json.c (lisp_to_json_toplevel_1): Serialize plists to json.
(Fjson_serialize): Mention plists in docstring.
(enum json_object_type): Add json_object_plist
(json_to_lisp): Parse JSON into plists.
(json_parse_object_type): Consider plists.
(Fjson_parse_string): Mention plists in docstring.
(syms_of_json): New Qplist sym_of_json

* test/src/json-tests.el (json-serialize/object)
(json-parse-string/object): New plist tests.