Lint `TRUE`, `nUlL`, etc., for Phabricator conventions

Summary: Detect and fix unconventional spellings of `true`, `false`, `null` and `array` (these are the only keywords I've seen spelled unconventionally in the wild).

Test Plan: Unit tests.

Reviewers: DurhamGoode, btrahan, vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T2985

Differential Revision: https://secure.phabricator.com/D5686