Modernize repository

[remcotolsma]

• [x] GitHub ⭐️
• [x] GitHub keywords
• [x] GitHub master → main
• [x] GitHub branch develop delete
• [x] Use pronamic/wp-coding-standards
• [x] Delete Grunt.js use
• [x] Use pronamic/pronamic-cli
• [x] Use https://github.com/Automattic/jetpack-autoloader