Ditch `.npmignore` in favor of `files` option in `package.json`

[MartijnCuppens]

Currently the .github folder, .editorconfig, .eslintignore, and .eslintrc.json are published on npm.

Instead of blacklisting the files you don't want to publish, it's easier to whitelist the ones you do want to be published. You can do this with the files option

[adamwathan]

This is definitely a good idea and would accept a PR that set this up for sure 👍 Thanks @MartijnCuppens!