Add codecov integration

[stefcameron]

Also adds missing peer dependency on @testing-library/dom for
@testing-library/user-event added in #222.

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• Issue being fixed is referenced.
• Source changes maintain:
  • Stated browser compatibility.
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• Unit test coverage added/updated.
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⚠️ No Changeset found

Latest commit: 04f4c17

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@5983c49). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #227   +/-   ##
=========================================
  Coverage          ?   97.14%           
=========================================
  Files             ?        1           
  Lines             ?       70           
  Branches          ?       26           
=========================================
  Hits              ?       68           
  Misses            ?        2           
  Partials          ?        0           

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[stefcameron]

I posted this PR before updating the first set of coverage (i.e. base line) from current master so Codecov couldn't report something meaningful here, but we should be good to go for future PRs.