Make the TodoComment Sniff to require a complete regex (start, end delimiters)

[stronk7]

While working on a new release of the moodle-plugin-ci including the latest version of moodle-cs, it has been detected that there is a small discordance between the TodoComment sniff (and its commentRequiredRegex property/config) and the FileExpectedTags (and its preferredLicenseRegexproperty/config):

• The TodoComment ones don't require the usual regex delimiters and adds ~ automatically.
• The FileExpectedTags ones do require a complete regex with delimiters.

I think that the later is better, so this issue is about to change the former to also require a complete regex with delimiters.