Chore: introduces core ES module.

[gjanblaszczyk]

Description

The PR introduces an alternative ES module file for videojs core build.
I created this PR because I would like to use videojs core directly from the browser like this:

<script type="module"> import videojs from 'video.js/dist/alt/video.core.es.js'; </script>

For now, main videojs build supports umd, cjs and es module. Not sure why core build supports only cjs and umd.

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Change has been verified in an actual browser (Chome, Firefox, IE)
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
• Reviewed by Two Core Contributors