Lack of `.json` extension prevents webpack from loading

[brianium]

Importing ./data/all from index.js doesn't work with webpack. Tacking on .json allows this to work with webpack, and should still work in node envs.

[vio]

@brianium Can you please elaborate on your use case? My guess is that your resolve.extensions is not containing .json anymore.

While the statement is ok for node module resolving, I am aware that there are issues when used in other environments that are not having the same resolving behavior (eg: jestjs/jest#3252).

Adding the extension should avoid this issues, please feel free to create a PR ;)