breaking: replace dependency opn w/ open

[erisu]

Motivation and Context

• Remove npm warning

npm WARN deprecated opn@6.0.0: The package has been renamed to open

Description

• Drop opn dependency
• Add open dependency
• Rebuilt package-lock.json

Testing

• npm t

Checklist

• I've run the tests to see all new and existing tests pass

[codecov-io]

Codecov Report

Merging #30 into master will not change coverage by %.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #30   +/-   ##
=======================================
  Coverage   27.98%   27.98%           
=======================================
  Files           6        6           
  Lines         218      218           
=======================================
  Hits           61       61           
  Misses        157      157           

Impacted Files	Coverage Δ
src/browser.js	18.39% <100.00%> (ø)

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