Modify `phutil_json_decode` to use `PhutilJSONParser`.

Summary: Ref T5297. Modify `phutil_json_decode` to use `PhutilJSONParser` instead of the native `json_decode`.

Test Plan: Ran `arc unit`.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5297

Differential Revision: https://secure.phabricator.com/D9634