Update setup

[mae829]

• Clean up files to abide by WP code standards
• Add nonce field and nonce check to abide by WP code standards
• Add check for autosave in save method
• Update gulp file for gulp 4
• Switch gulp file name to use babel and ES6
• Add a gulp config file holding sever config
• Update workflow to use stylelint + webpack
• Organize files into src/assets