Add validation for since tag version

[roykho]

This PR adds validation for the [at]since version to make sure they have supplied a valid version for that tag.

Closes #28

Prereq:
Make sure you have this PR pulled down and that your global PHPCS is referencing this change.

Testing:

• Git clone https://github.com/woocommerce/woocommerce.
• Go to your cloned repo and modify the file plugins/woocommerce/woocommerce.php. Add a do_action or apply_filters hook with docblock. Add the [at]since tag without version. For example:

/**
 * Description
 *
 * @since
 */
do_action( 'hook' );

• Now in the root of your repo run phpcs -s plugins/woocommerce/woocommerce.php.
• Ensure you see the violation that no versions were supplied with the [at]since tag.
• Now add the version in either <x.x.x> or <x.x> format and run the sniff command again.
• This time you should see no violations.

Note:
I did not add the check for description as per the original #28 issue as this is already being picked up by the generic PEAR rules.

Task:
After approved and merged, we need to do a release and bump the woocommerce-sniffs version in WooCommerce-monorepo.

[psealock]

Make sure you have this PR pulled down and that your global PHPCS is referencing this change

What's the best way to do this? Using the --standard=path/to/standard isn't working for me as it can't find the WooCommerce ruleset

# From this repo
phpcs --standard=src/WooCommerce-Core ../woocommerce/plugins/woocommerce/woocommerce.php

[roykho]

@psealock - I would suggest installing it globally. composer global require woocommerce/woocommerce-sniffs. Then you can run phpcs --config-set default_standard WooCommerce-Core. Then find where your phpcs rulesets are installed by running phpcs --config-show. You can then symlink this repo to where you have it installed so your global will always reference this repo for development.

[psealock]

Nice job @roykho ! This is working great, and a missing description is picked up as a violation as well 🚢