Gulp 4.x - Node.js LTS 12.x support

Breaking changes

• email-fix task was renamed to emailfix. To keep backward compatibility you can still use old naming via npm scripts yarn email-fix or npm run email-fix
• AT themes.json in postcss config of the theme, you shouldn't use plugins.something anymore, but just something. Although, all dependencies are no longer automatically available everywhere, so if you wish to use some additional PostCSS plugin, you need to import it manually in helpers/scss.js file. Please check sample config

Under the hood changes

• Rewrite to Gulp 4.x - ES Modules, cleaner code and other cool stuff
• Node.js 12.x (active LTS) support
• Dependencies clean up and update, including vulnerability fixes