Add encodeAsList to PhutilJSON and make formatting prettier
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

julkiewicz/libphutil,julkiewicz/libphutil,julkiewicz/libphutil