Add encodeAsList to PhutilJSON and make formatting prettier

Summary:
Ref T8973. Ref T7419. Two issues:

• In documenting T7419, I want to pretty print a JSON list value (an example value for lint and unit when calling harbormaster.sendmessage). Expose a public method for this.
• T8973 discusses PHP's default behavior of escaping forward slashes in JSON. This is legal, but cumbersome. When pretty-printing for human users, disable it if we have a recent-enough PHP version.

Test Plan:

• See next diffs.
• Pretty-printed lists with values that contain slashes, got friendly/helpful results:

{F688892}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7419, T8973

Differential Revision: https://secure.phabricator.com/D13787