Bundles in dist have no extension

Hey there. I work for a company that inspects and analyzes npm packages from the registry. We noticed today that you're shipping files in `dist` that have no file extension. This confuses most bundlers and platforms that are not node, which don't have a loose `require` process/algorithm.

We'd like to suggest that you add a file extension to `dist/cjs` and `dist/esm`. Cheers