Review sniff handling of PHP 7.4 arrow functions

[jrfnl]

PHP 7.4 introduces short closures/arrow functions.

The WPCS native sniffs will need to be reviewed to make sure that they handle those correctly.

• Any sniff which examines function declarations will need the T_FN token added.
• Any sniff which examines the => T_DOUBLE_ARROW token will need unit tests added to make sure the double arrow in arrow functions is handled correctly or ignored, depending on the purpose of the sniff.
• ... tbd ...

Refs:

• PHP RFC containing lots of code samples: https://wiki.php.net/rfc/arrow_functions_v2
• Upstream issue about arrow functions: PHP 7.4: arrow functions squizlabs/PHP_CodeSniffer#2523

Related to #764

[jrfnl]

As of PHPCS 3.5.3, PHPCS backfills the T_FN token and will tokenize the => for arrow functions as T_FN_ARROW.

[jrfnl]

PHPCSUtils contains a few utilities for recognizing arrow functions correctly for those PHPCS versions which don't properly support them yet (=all currently released versions. Support was added in 3.5.3, but very buggy and the latest bug fixes haven't been released yet)

[jrfnl]

All sniffs have been reviewed and fixed to support modern PHP in as far as my imagination reached.

If any more situations would be discovered which need to be taken into account, a new issue should be opened with code samples etc.

Closing this now as fixed.