PHP 8.0 | New NewAttributes sniff

[jrfnl]

PHP 8.0 introduces Attributes as a form of structured, syntactic metadata to declarations of classes, properties, functions, methods, parameters and constants. Attributes allow to define configuration directives directly embedded with the declaration of that code.

The syntax for attributes in PHP has changed a number of times via various RFC amendments.
As a result of that, it took a little while as well before the final tokenization of attributes in PHPCS was known, but upstream PR squizlabs/PHP_CodeSniffer#3203 has now (finally) been merged and released as part of PHPCS 3.6.0.

This PR introduces a new sniff to detect the use of attributes.

This new sniff only focusses on detecting the syntax for attributes being used - #[...].
It does not verify whether the contents within the attribute is valid, nor whether the attribute is used in a supported location.
Invalid attributes like that would be a parse error in PHP 8.0+.

It can be argued that single-line attributes should not throw an error, but a warning, as those will not necessarily cause problems when used in code which also needs to run on PHP 7, as they will just be ignored and treated as a comment.
This is however not the case for multi-line or in-line attributes, which would cause parse error in PHP < 8.0, so for consistency, I've chosen to let the sniff always throw an error.

In rare cases, specifically for the code sample in the last test case, the PHPCS tokenizer cannot prevent the rest of the file becoming mangled on PHP < 8.0.
This is similar to the tokenizer problems caused by indented heredocs/nowdocs as introduced in PHP 7.3, where the rest of the file after the new syntax will be tokenized incorrectly.
In that case, the sniff will still (correctly) throw an error for the attribute, however detection of (other) issues in the rest of the file will be broken.
Also see: squizlabs/PHP_CodeSniffer#3294

As the attributes feature is likely to be expanded in the future, the sniff has been placed in a new dedicated category which will allow for additional attribute related sniffs to be added at a later point in time.

If and when new attribute related features are added to PHP, validation of whether an attribute is valid in PHP 8.0 may need to be added to this sniff, but this is a moot point until that time.

Includes unit tests.

Refs:

• https://wiki.php.net/rfc/attributes_v2
• https://wiki.php.net/rfc/attribute_amendments
• https://wiki.php.net/rfc/shorter_attribute_syntax
• https://wiki.php.net/rfc/shorter_attribute_syntax_change
• https://www.php.net/manual/en/language.attributes.php

Related to #809

[jrfnl]

Thanks for reviewing critically @elazar ! I much rather have questions which turn out to be already covered, than that something is missed in the PR, so I really appreciate your critical look.