Fix importing rdg with webpack 4

[nstepien]

• webpack 4 doesn't treat .mjs files as esm by defeault for some reason, so we have to publish the esm bundle as .js
  • .mjs and ESM interop with module main, and transpiling to CJS webpack/webpack#7482 (comment)
• In node, importing react-data-grid from an esm only works if its file extension is .mjs, or .js when "type": "module" is in the package.json
• Now that we use "type": "module", modules that are imported from commonjs modules must have the .cjs file extension, as cjs cannot import esm in Node.
  • this is why eslint and storybook had to be fixed.
  • all .js files are treated as esm now by Node.
• Importing esm rdg in Node will not work because Node cannot find the react/jsx-runtime import without a file extension, but that could be fixed with react adding the exports field.