Prep Plugin for release on WordPress.org (#23)

* Update translation text domain
* Escape everything that should be escaped.
* Add nonce checks where needed.
* Sanitize all inputs.
* Apply Code style changes across the codebase.
* Correct many deprecation notices.
* Optimize load order of many filters.
* Add Proper Build script
* Use emojii flags
* Fix i18n deprecation  notice for translating too early
* Put all scripts in footer and load async

views/checkout/templates/steps/legacy.php

@@ -38,7 +38,7 @@ if ( ! defined('ABSPATH')) {
 			$class = 'active';
 		} elseif (array_search($current_step, array_column($steps, 'id')) > array_search($step_key, array_column($steps, 'id'))) {
 			$class = 'done';
-		} // end if;
+		}
 
 		?>