Create package.json for esm and cjs distributions

[lmachens]

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I faced the issue that I could not import openapi-typescript-fetch as ESM. After some research, I think it's related to missing package.json to specify type in the distributions. See https://www.sensedeep.com/blog/posts/2021/how-to-create-single-source-npm-module.html for an example.

This pull request adds package.json in the dist folders with module commonjs/module.