Added cross-env to ensure NODE_ENV works on Windows

[marianban]

Hi,

I added cross-env so the package can be built on windows.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@            Coverage Diff             @@
##             master      #166   +/-   ##
==========================================
  Coverage          ?   100.00%           
==========================================
  Files             ?         9           
  Lines             ?        63           
  Branches          ?        12           
==========================================
  Hits              ?        63           
  Misses            ?         0           
  Partials          ?         0           

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