Add constants and class methods to `php_compat_info.json`.

Summary: Allows the `ArcanistXHPASTLinter` to determine whether constants can be used, based on the target PHP version. Also added class methods to the compatibility information, although this isn't used yet (it is small anyway).

Test Plan: Created a test file that contained the `JSON_PRETTY_PRINT` constant. Verified that a linter error was raised.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9571