esModule: false fix read only TypeError in exports

[cif]

As suggested from previous PR, any client exclusively using react-router-dom package via CommonJS modules will run into this problem:

    TypeError: Cannot assign to read only property '__esModule' of object '[object Object]'

      at node_modules/react-router-dom/cjs/react-router-dom.js:288:64
          at Array.forEach (<anonymous>)
      at Object.<anonymous> (node_modules/react-router-dom/cjs/react-router-dom.js:288:26)
      at Object.<anonymous> (node_modules/react-router-dom/index.js:6:20)

Changing rollup config to omit Object.defineProperty(exports, '__esModule', { value: true }); will fix this, however my suggestion in #6757 seems less heavy handed in that it doesn't break interop https://stackoverflow.com/a/55687758

[cif]

Maybe interop wasn't the best idea anyway¯_(ツ)_/¯ https://medium.com/@dlmanning/interoperability-between-es-modules-and-common-js-is-a-mistake-fb9ac71d96fd

[timdorr]

OK, I'm down with this.

One thing: Can you actually run the build and update the bundle size snapshots? This will change them.

[cif]

When I ran npm build to test locally that didn't seem to produce any other artifacts. I just went through and deleted the .size-snapshot.json files, re-ran npm build and they appeared to produce unchanged versions of master, so there's nothing to commit. Without diving in more, seems the way the reports are configured the single line difference in CJS export isn't turned up?

[timdorr]

Oh duh, this is only for the CJS build, which isn't tracked by that file. We're all good then.

[cif]

Awesome thank you! This was blocking us from updating to v5 on a new project using Jest. I'll hang back on 4 until it's released but hopefully that does the same for others.

Really appreciate the hard work and dedication to OSS projects like this one @timdorr !