Add the Generic.PHP.BacktickOperator rule

[aadmathijssen]

This PR disallows the use of the backtick operator by adding the Generic.PHP.BacktickOperator rule.

Consider the following confusing PHP code:

<?php

declare(strict_types=1);

echo '`ls`';
echo `'ls'`;

With the current ruleset, this does not report any error.

After merging this PR, the following error is reported:

FILE: /path/to/file.php
--------------------------------------------------------------------------------------------
FOUND 2 ERRORS AFFECTING 1 LINE
--------------------------------------------------------------------------------------------
 6 | ERROR | Use of the backtick operator is forbidden
   |       | (Generic.PHP.BacktickOperator.Found)
 6 | ERROR | Use of the backtick operator is forbidden
   |       | (Generic.PHP.BacktickOperator.Found)
--------------------------------------------------------------------------------------------

To repair this error, you can use the shell_exec function instead, that does not require a potentially confusing syntax:

<?php

declare(strict_types=1);

echo '`ls`';
echo shell_exec('ls');