Functions/StripTags: various sniff improvements

[jrfnl]

Functions/StripTags: improve the tests

• Test against false positives for calls to methods or namespaced function calls.
• Test against false positives for attribute class using the same name as the function.
• Ensure function import use statements are not flagged. We're not interested in those.
• Add some more variations to the pre-existing tests:
  • Non-lowercase function call(s).
  • Fully qualified function calls.
  • Use PHP 7.3+ trailing comma's in a few function calls.

Includes cleaning up some "Warning" comments which contained irrelevant information as the sniff isn't looking for the things which were previously mentioned.

Functions/StripTags: always flag use of the function

This function should never be used, so should always be flagged, no matter whether the code is (not yet) valid.

Realistically, the existing StripTagsOneParameter error code should be replaced with the new Used error code, but that would be a breaking change, which would have to wait for the next major (if deemed worth it).

I checked via a code search and the error code as-is, is referenced in various code bases, so I'm maintaining BC for now.

Functions/StripTags: document handling of PHP 5.6 argument unpacking

This is flagged under the StripTagsOneParameter error code.

Functions/StripTags: add support for handling PHP 8.0+ function calls using named parameters

This does change the logic of the sniff a little as well, as if there are parameters, but all of those would use incorrect parameter names, we should still want the function call to be flagged.

Includes tests.

Functions/StripTags: add support for handling PHP 8.1 first class callables

First class callables are basically syntax sugar for closures.
So:

add_action('my_action', strip_tags(...));

... could also be written as:

add_action('my_action', function(...$args) {
    return strip_tags(...$args);
});

Example: https://3v4l.org/cra8s#veol

As the code would be flagged when used in a closure, it is my opinion, it should also be flagged when used as a first class callable.

This commit implements this in a WPCS cross-version compatible manner.

Prior to WPCS 3.2.0, first class callables would be send to the process_no_parameters() method.
As of WPCS 3.2.0, they will be send to the dedicated process_first_class_callable() method. See WordPress/WordPress-Coding-Standards#2544

Closes #519