Add npm run watch

[jednano]

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[codecov]

Codecov Report

Merging #6 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master     #6   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines          32     32           
  Branches       10     10           
=====================================
  Hits           32     32

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