[PSR-12] Property visibility

Disclaimer: I am aware that it's not make-or-break for Prettier to adhere to PSR-12.

However, now that it is officially accepted as a standard, we should discuss (and, if necessary, reject) each current violation of PSR-12 — if only to have point to refer to in future discussions or issues.

PSR-12 Violations

1. Visibility MUST be declared on all properties.
   – Section 4.3

2. Visibility MUST be declared on all constants if your project PHP minimum version supports constant visibilities (PHP 7.1 or later).
   – Section 4.3

   Solving this would need an --engine flag.

PSR-12 Section 4.3 と issue #939 にあるリンク先の --engine に関する議論から始めてください。サポート対象のすべての PHP バージョンにおけるプロパティと定数の可視性について、意図された動作を特定してください。このスコープと実装アプローチについて合意されて初めて、issue は完了となります。