New sniff: add visibility to class constant declarations

[jrfnl]

Sniff basics	-
Fixable for PHP:	7.1+
Sniff type:	Modernize
Fixer type:	Safe (review recommended)

Short description of the sniff

Prior to PHP 7.1, class constants could not have visibility declared and would always be public. As of PHP 7.1, there is a choice between declaring class constants as public, protected or private.

Related PHPCompatibility sniff(s):

• NewConstVisibility

PHP manual references:

• As of PHP 7.1.0, class constants may be defined as public, private, or protected. Constants declared without any explicit visibility keyword are defined as public.

  http://php.net/manual/en/language.oop5.visibility.php#language.oop5.visiblity-constants

• http://php.net/manual/en/migration71.new-features.php#migration71.new-features.class-constant-visibility
• http://php.net/manual/en/language.oop5.constants.php

Example code:

Detect the following code pattern(s):

class ClassConstants
{
    const MYCONST_A = 1;
}

And fix these to:

class ClassConstants
{
    public const MYCONST_A = 1;
}

Notes for implementation of the sniff:

• This is only possible for class constants, so the scope of the const keyword will need to be carefully checked.