Add eslint-plugin-import resolvers for node and webpack

This commit adds two npm packages that support [eslint-plugin-import][].

[eslint-plugin-import]: https://github.com/benmosher/eslint-plugin-import

- [eslint-import-resolver-webpack][]
- [eslint-import-resolver-node][]

[eslint-import-resolver-webpack]: https://www.npmjs.com/package/eslint-import-resolver-webpack
[eslint-import-resolver-node]: https://www.npmjs.com/package/eslint-import-resolver-node

More details on how [eslint-plugin-import][] implements resolvers can be
found [in its README][resolvers].

[resolvers]: https://github.com/benmosher/eslint-plugin-import#resolvers