Lint JSON files with `ArcanistJSONLinter`.

Summary: After D9628, we can lint JSON files natively.

Test Plan: See D9652.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9653
Update library map for `phutil_is_utf8_slowly`.

Summary: It appears that `arc liberate` wasn't run for D9472.

Test Plan: Meh...

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9665
Minor whitespace changes.

Summary:
A few minor whitespace changes:

- Change tabs to spaces.
- Don't change tabs to spaces in test cases.
- Don't apply `.editorconfig` rules to the `externals` directory.

Test Plan: N/A

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9673