Add CommonJS export

[sgwilym]

Hello! I am using this dependency to help me build an isomorphic NPM module which has both ESM and CommonJS distributions. I wasn't able to do this without the addition of a CommonJS export to this package, which is currently ESM only.

• Added index.node.cjs
• Renamed index.node.js to index.node.mjs
• Added import and require conditions to the node export in package.json
• Changed package.json's main to point at the .cjs, and added a module key which points at the .mjs variant

[Swimburger]

Thank you for this PR. I went ahead and forked the repo and pulled in your PR to resolve an issue I had with using this with Jest. I hope this is merged into this repo too tho.