Bundles in dist have no extension

[shellscape]

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