Add files field to limit files installed when installed as a package

[openam]

Since the utils is included when I install this package I'm getting some vulnerabilities showing up in some scans. I know they are not used via the NPM package so it would be nice to just exclude them from the installed package.

$ npm pack --dry-run                                                            
npm notice 
npm notice 📦  [email protected]
npm notice === Tarball Contents === 
npm notice 1.1kB   LICENSE                  
npm notice 6.2kB   README.md                
npm notice 43.9kB  data/ca-cities.json      
npm notice 314B    data/ca-states.json      
npm notice 384.7kB data/cities.json         
npm notice 1.1kB   data/states.json         
npm notice 340.8kB data/us-cities.json      
npm notice 1.2kB   data/us-states.json      
npm notice 14.5kB  data/us-street-types.json
npm notice 13.9kB  index.js                 
npm notice 889B    package.json             
npm notice === Tarball Details === 
npm notice name:          addresser                               
npm notice version:       1.1.19                                  
npm notice filename:      addresser-1.1.19.tgz                    
npm notice package size:  292.0 kB                                
npm notice unpacked size: 808.6 kB                                
npm notice shasum:        cae204a2d17f8d881d181318f3bd15bcdae1ad10
npm notice integrity:     sha512-zuzIV/VDji+DS[...]lpdGCE6JvpkFQ==
npm notice total files:   11                                      
npm notice 
addresser-1.1.19.tgz

Vulnerability scan findings: