fix: change rollup config so that npm run watch works

[squarebracket]

Description

Currently, when running npm run watch, editing a file will cause a failure in generating the CommonJS and ES outputs:

bundles src/js/index.js → dist/video.es.js, dist/video.cjs.js...
[!] Error: Could not load mux.js/lib/tools/mp4-inspector (imported by /tmp/video.js/node_modules/@videojs/http-streaming/dist/videojs-http-streaming.es.js): ENOENT: no such file or directory, open 'mux.js/lib/tools/mp4-inspector'
Error: Could not load mux.js/lib/tools/mp4-inspector (imported by /tmp/video.js/node_modules/@videojs/http-streaming/dist/videojs-http-streaming.es.js): ENOENT: no such file or directory, open 'mux.js/lib/tools/mp4-inspector'
    at /tmp/video.js/node_modules/rollup/dist/rollup.js:17372:19
    at <anonymous>

Specific Changes proposed

Add mux.js/lib/tools/mp4-inspector to the list of externals.

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