Maintain ordering of JSON object keys by default

Maintain ordering of JSON object keys by default

* lisp/json.el (json-object-type): Mention order handling in doc-string.
(json--plist-reverse): New utility function.
(json-read-object): Maintain ordering for alists and plists.
(json-pretty-print): Ensure that ordering is maintained.

* test/automated/json-tests.el (test-json-plist-reverse): New test for
`json--plist-reverse'.
(json-read-simple-alist): Update test to accommodate for changes in
`json-read-object'.

* etc/NEWS: Document the new behavior of the pretty printing functions.