fix(#584): add files key to package.json

[doug-wade]

Fixes #584

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (71bde0c) 89.32% compared to head (04128f1) 89.32%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #593   +/-   ##
=======================================
  Coverage   89.32%   89.32%           
=======================================
  Files          53       53           
  Lines        1133     1133           
  Branches      205      205           
=======================================
  Hits         1012     1012           
  Misses        121      121           

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