fix #26, modernize pkg config

[codejedi365]

This PR provides:

1. Fix to a modern use of require(), which doesn't fail on node@>=10.24.
2. I noticed from the flowconfig that it only defines index.js as the single file of code needed for the module. This same construct is available via files directive in package.json.
3. The previous alternative to the files directive is the definition of .npmignore. I updated this to solve the issue in case files directive is not desired. The do not conflict with each other and files array takes precedence.

Resolves #26

[codejedi365]

@loganfsmyth, any thoughts?