Make npm package smaller - 720 bytes after unpacking

Make npm package smaller by skipping some files #4

$ npm publish --dry-run
npm notice 
npm notice 📦  @automattic/escapeshellarg@0.2.0
npm notice === Tarball Contents === 
npm notice 135B lib/escapeshellarg.js
npm notice 281B package.json         
npm notice 304B README.md            
npm notice === Tarball Details === 
npm notice name:          @automattic/escapeshellarg              
npm notice version:       0.2.0                                   
npm notice package size:  535 B                                   
npm notice unpacked size: 720 B  

Publish as @automattic/escapeshellarg

• Fix bug where single quotes are escaped incorrectly #1 (by @delan)
• Add CI workflow #2
• Automated publishing to npm #3