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

assets/js/flags.js

@@ -0,0 +1,2 @@
+import { polyfillCountryFlagEmojis } from "https://cdn.skypack.dev/country-flag-emoji-polyfill";
+polyfillCountryFlagEmojis();