chore: make CI run as non-root

[jinwoo]

Hopefully this will fix the auto-NPM-publishing. Currently the prepare
phase fails.

[codecov-io]

Codecov Report

Merging #20 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #20   +/-   ##
=======================================
  Coverage   81.57%   81.57%           
=======================================
  Files           4        4           
  Lines         331      331           
  Branches       70       70           
=======================================
  Hits          270      270           
  Misses         61       61

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[jinwoo]

Piggybacking the version update change so that I can publish new NPM right after this.

[jinwoo]

Also adding the files stanza to package.json so that unnecessary files are not included in the NPM package.

[kjin]

LGTM