Add code coverage workflow

[cjbarth]

Description

Add support for automatically running code coverage reports when making a PR.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@9dac01f). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #706   +/-   ##
=========================================
  Coverage          ?   65.10%           
=========================================
  Files             ?        4           
  Lines             ?      149           
  Branches          ?       37           
=========================================
  Hits              ?       97           
  Misses            ?       29           
  Partials          ?       23           

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