Add sniff to disallow unused variables

[aadmathijssen]

This PR adds the SlevomatCodingStandard.Variables.UnusedVariable sniff, which detects unused variables. See the unusedVariableErrors.php test file for a number of examples that are detected by this tool.

For key/value pairs in a foreach statement where only the key is used, no violation is reported when the value is unused (this required setting the ignoreUnusedValuesWhenOnlyKeysAreUsedInForeach property to true).

So the following file does not generate an error:

<?php

declare(strict_types=1);

function keystoupper(array $values): array
{
    $result = [];
    foreach ($values as $key => $value) {
        $result[] = strtoupper($key);
    }
    return $result;
}

Note that the following still generates an error, because the key is unused:

<?php

declare(strict_types=1);

function strstoupper(array $values): array
{
    $result = [];
    foreach ($values as $key => $value) {
        $result[] = strtoupper($value);
    }
    return $result;
}

The following error is reported:

FILE: /path/to/file.php
----------------------------------------------------------------------------------------------------
FOUND 1 ERROR AFFECTING 1 LINE
----------------------------------------------------------------------------------------------------
 8 | ERROR | Unused variable $key.
   |       | (SlevomatCodingStandard.Variables.UnusedVariable.UnusedVariable)
----------------------------------------------------------------------------------------------------

NB This PR upgrades the Slevomat Coding Standard to v7.0.19, which fixes a few issues in this sniff (#1320 and #1321).