Make minified source available to all tools

[david-bezero]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and couldn’t find anything (or linked relevant results below)
• If applicable, I’ve added docs and tests (not applicable)

Description of changes

Support other tools which cannot work with ES Modules (e.g. Jest). The necessary code is already available but using a proprietary key in package.json (for unpkg). This adds standards-compliant config to make it available to other tooling which needs it.

[david-bezero]

the test failure seems to be because some build tooling is incompatible with the latest change to (experimental) loaders in Node 18.6 (unrelated to this change)

[ChristianMurphy]

There is a dual package hazard when exporting both CJS and ESM.
It has been discussed at some length at remarkjs/remark#969 (comment) and unifiedjs/unified#121 (comment)
It can cause real problems, trying to work around Jest-specific limitations.

There are some suggestions on how to make Jest play nicely with ESM in #635

[david-bezero]

@ChristianMurphy I guess note that the dual package hazard only applies to packages which contain meaningful global state (which, as far as I can see, this one doesn't). Also you're already providing the dual packages; this PR just makes them both available to the tooling, so I hope you might reconsider this decision.

For now, the only workaround available to users is this rather nasty config:

  moduleNameMapper: {
    'react-markdown': '<rootDir>/node_modules/react-markdown/react-markdown.min.js',
  },

[ChristianMurphy]

The dual package hazard has several pitfalls and risks, global state is just one of them.
In addition, some remark/rehype plugins do use global state.

Having to configure around a bug/limitation of Jest is an acceptable trade-off, vs the risks and known issues of dual packaging.
And I'm hopeful in the future Jest will make ESM fully supported and seamless jestjs/jest#9430