Add a new moodleTodoCommentRegex config option

While everything continues working the same and the
Sniff property (`commentRequiredRegex`) still is available
and defaulting to `MDL-[0-9]+`, now we can, also, from command
line, decide which regular expression to use.

That will make things easier from other tools, not having
to modify the ruleset phpcs.xml files. Instead just run:

vendor/bin/phpcs --runtime-set moodleTodoCommentRegex 'CONTRIB-[0-9]+'

or any other valid regular expression (https, github urls...)

In order to disable the Sniff, you can pass to it an empty string
(apart from excluding it completely if desired, of course)

vendor/bin/phpcs --runtime-set moodleTodoCommentRegex ''

moodle/Sniffs/Commenting/TodoCommentSniff.php

@@ -29,6 +29,7 @@
 
 // phpcs:disable moodle.NamingConventions
 
+use PHP_CodeSniffer\Config;
 use PHP_CodeSniffer\Files\File;
 use PHP_CodeSniffer\Sniffs\Sniff;
 
@@ -71,6 +72,11 @@ public function register(): array {
      * @return void
      */
     public function process(File $phpcsFile, $stackPtr): void {
+        // If specified, get the regular expression from the config.
+        if (($regex = Config::getConfigData('moodleTodoCommentRegex')) !== null) {
+            $this->commentRequiredRegex = $regex;
+        }
+
         // If the regular expression is empty, then we don't want to do anything.
         if (empty($this->commentRequiredRegex)) {
             return;