Publish CommonJS dist files

[sqs]

Because this package only publishes/distributes ES6 modules, tooling that expects all node_modules/**/*.js files to be CommonJS modules will break. There are simple workarounds for this issue (see https://github.com/sourcegraph/sourcegraph/pull/1456#discussion_r241994980), so this is not high priority.

(Related issue for lodash-es: https://github.com/facebook/create-react-app/issues/5241#issuecomment-426269242)