Introduce codecov

[liuderchi]

introduce codecov following https://github.com/codecov/example-node#jest

Steps to introduce codecov

• [x] [Report] for repo using jest, just add jest field config to tell jest generate coverage report after test
• [x] [CC] get token from codecov.io
• [x] [CC] install codecov github integration https://github.com/apps/codecov (need set permission)
• [x] [Report] yarn add -D codecov and run jest && codecov -t :codecov-token
  • generate then upload first coverage report to activate your repo in codecov
• [x] [GH] commit then create PR, codecov bot will show report for you
• [x] [GH] tweak jest config for correct coverage (_Example)

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@4efaa44). Click here to learn what that means. The diff coverage is n/a.

@@           Coverage Diff            @@
##             master     #12   +/-   ##
========================================
  Coverage          ?   74.5%           
========================================
  Files             ?       2           
  Lines             ?      51           
  Branches          ?      11           
========================================
  Hits              ?      38           
  Misses            ?      10           
  Partials          ?       3

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