Get markers data from helper

[Coosos]

• Use of a helper to be reused by store delivery module
• Use magento stdlib datetime instead of Zend_Locale_Format
• Call once to the getRetailerCollection method

I fixed the PHPMD & PHPCS errors regarding modified files

We must adapt the code to make it compliant 1.x

[fnogatz]

The commit https://github.com/Smile-SA/magento2-module-store-locator/pull/118/commits/03500d89bf9b46689ea8684b52674663940accbc, Use stdlib timezone instead of zend format time, is also required to make Smile_StoreLocator working with the most recent version v2.4.6 of Magento, as it deprecated Zend framework (ZF1) components (see Release Notes).