Start using WordPressCS 3.1

[jrfnl]

Composer: update to WordPressCS 3.

Composer:

• No need for requiring the Composer PHPCS plugin anymore as it now comes automatically with WPCS and removing the requirement allows WPCS to manage the supported versions, preventing conflicts (and will update to the 1.0.0 version).

PHPCS ruleset:

• Minor tweak for a replaced sniff as per the changes in WPCS 3.0.0.
• Exclude one error code which would conflict with the minimum PHP requirement of this plugin.

Refs:

• https://github.com/WordPress/WordPress-Coding-Standards/releases/tag/3.0.0
• https://github.com/WordPress/WordPress-Coding-Standards/wiki/Upgrade-Guide-to-WordPressCS-3.0.0-for-ruleset-maintainers

CS/QA: all methods should have visibility declared

To prevent any BC-breaks, all methods without explicit visibility have now been explicitly declared as public.

Mind: the set_up() and tear_down() methods in the tests should be protected, but as WP Core didn't always have this correct and this plugin still supports a large range of WP versions, so needs to take a large range of WP versions into account for the WP Core test framework on which the test suite is build, setting the visibility to public is the safer choice for now.

CS/QA: all properties should have visibility declared

To prevent any BC-breaks, all properties without explicit visibility have now been explicitly declared as public.

CS: always use parentheses for class instantiation

CS/QA: don't use reserved keywords as parameter names

... to prevent code getting really confusing when people would call it using PHP 8.0+ function calls with named arguments.

Modernize: use the magic DIR constant

CS/QA: miscellaneous other fixes

• Use pre-increment instead of post-increment.
• Use require for unconditional file includes.
• No blank line at end of function.