3.0: Move "variable" related utilities to dedicated VariableHelper + use PHPCSUtils + handle match

[jrfnl]

Move "array access keys" related utilities to dedicated VariableHelper

The "array access keys" related utilities are only used by a small set of sniffs, so are better placed in a dedicated class.

This commit moves the get_array_access_keys() method and the get_array_access_key() method to a new WordPressCS\WordPress\Helpers\VariableHelper and starts using that class in the relevant sniffs.

In contrast to some of the other "move methods out of the Sniff class" PRs, these methods have been moved to a class and made static - instead of moved to a trait.
The reason for this difference is that the methods in other "moves" are setting properties which the sniff classes would need access to, while these methods are 100% stand-alone.

Note:
It is expected for PHPCSUtils to have dedicated methods for the same at some point in the future. If/when those methods become available, it is recommended for the sniffs to start using the PHPCSUtils methods. With this in mind, this class has been marked as "internal" without BC promise.

Related to #1465

VariableHelper::get_array_access_keys() use PHPCSUtils

Use the PHPCSUtils GetTokensAsString::compact() method to retrieve array access keys without extraneous whitespace or comments.

Move "is comparison" related utility to dedicated VariableHelper

The "is comparison" related utility method is only used by a small set of sniffs, so is better placed in a dedicated class.

As this method also expects a T_VARIABLE token as input, the VariableHelper class seems appropriate.

This commit moves the is_comparison() method to the new WordPressCS\WordPress\Helpers\VariableHelper and starts using that class in the relevant sniffs.

Note:
It is expected for PHPCSUtils to have dedicated methods for the same at some point in the future. If/when those methods become available, it is recommended for the sniffs to start using the PHPCSUtils methods.

Related to #1465

VariableHelper::is_comparison(): use PHPCSUtils

... to get the potential parentheses opener.

PHP 8.0 | VariableHelper::is_comparison(): add support for variables being compared in a match expression

PHP 8.0 introduced match expressions, which do a (strict) comparison on the value in the condition against the options in the match body.
Ref: https://www.php.net/manual/en/control-structures.match.php

This commit updates the is_comparison() method to allow for variables in the "condition" part of a match expression to be considered part of a comparison.

Tested via the ValidatedSanitizedInput sniff.

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N.B. While reviewing the "array access keys" methods, I noticed that the methods do not account for array assignments without a key, i.e. $var[]. This may need improved handling at some point, but is outside the scope of this PR.

[GaryJones]

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