Add codecov GitHub Action

[hootener]

This Pull Request adds Codecov as a GitHub Action. I've verified that it works on a fork of this repo here:

• GitHub Actions output: https://github.com/hootener/php-library-skeleton/pull/1/checks?check_run_id=983229952
• Commits for this branch on Codecov: https://codecov.io/gh/hootener/php-library-skeleton/commits
• Forked PR with Codecov comment: https://github.com/hootener/php-library-skeleton/pull/1

Happy to answer any other questions :), and kudos on the 100% code coverage!

[codecov-commenter]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@855c4c7). Click here to learn what that means. The diff coverage is n/a.

@@            Coverage Diff             @@
##             master        #8   +/-   ##
==========================================
  Coverage          ?   100.00%           
  Complexity        ?       269           
==========================================
  Files             ?        48           
  Lines             ?      1009           
  Branches          ?         0           
==========================================
  Hits              ?      1009           
  Misses            ?         0           
  Partials          ?         0           

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