fix: Compile demo CSS/JS during npm run build

[acdvorak]

Fixes a regression introduced by #2325.

Supersedes #2433.

To verify:

1. Build and deploy to staging server:

   MDC_ENV=development npm run build && gcloud app deploy app.yaml --project google.com:mdc-web-dev --version $USER

2. View the staging server:

   http://$USER.material-components-web.appspot.com
   

[codecov-io]

Codecov Report

Merging #2437 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2437   +/-   ##
=======================================
  Coverage   98.86%   98.86%           
=======================================
  Files         101      101           
  Lines        4151     4151           
  Branches      531      531           
=======================================
  Hits         4104     4104           
  Misses         47       47

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[williamernest]

Builds and deploys fine. LGTM