Review sniff handling of PHP 7.4 numbers using numeric literal separator

[jrfnl]

PHP 7.4 introduces numeric literal separators.

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

As of PHPCS 3.5.3 the tokenization of numbers using separators is backfilled for older PHP versions, so I'd strongly recommend we update the minimum PHPCS requirement of WPCS to PHPCS 3.5.3 in the near future as it will be hard to handle this without the backfilled tokenization.

• Any sniff which examines T_LNUMBER and T_DNUMBER tokens is affected for the token walking part until PHPCS 3.5.3.
• If the numeric value of a T_LNUMBER of T_DNUMBER needs to be examined - like in the CronInterval sniff, but probably in a few more sniffs -, the numeric value of the $token['content'] will need to be cast to (int)/(float) before doing any comparison or calculations with that number.
• ... tbd ...

Refs:

• PHP RFC containing lots of code samples: https://wiki.php.net/rfc/numeric_literal_separator
• Upstream issue about numeric literal separators: PHP 7.4: backfill for tokenizing numeric sequences with an underscore separator squizlabs/PHP_CodeSniffer#2546

Related to #764

[jrfnl]

PHPCSUtils contains a few utilities for recognizing and dealing with numeric literals with underscores for those PHPCS versions which don't properly support them yet (= PHPCS < 3.5.4. Support was added in 3.5.3, but very buggy, I fixed that in 3.5.4)

[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.