Add .npmignore for non-production data

[Aras Abbasi]

It would be nice if we could add a .npmignore and remove all non-relevant files for the npm package.

Currently find-my-way is about 258 kb big. We could save 178 kb when we ignore

*
!package.json
!README.md
!LICENSE
!lib/
!index.js
!node.js
!index.d.ts

not tested