Add a native JSON parser.

Summary: Ref T5297. Add JsonLint as an external library and provide PhutilJSONParser as a wrapper around the library's functionality.

Test Plan: Wrote some basic unit tests and ran arc unit.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T5297

Differential Revision: https://secure.phabricator.com/D9622