Add github action to shellcheck master on push and PRs

[yarikoptic]

Currently it will fail with good number of errors etc to fix up. Let's see if workflow works and shows them.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 47.54%. Comparing base (6b7bda4) to head (06c73e0).
Report is 1 commits behind head on master.

❗ Current head 06c73e0 differs from pull request most recent head c319835. Consider uploading reports for the commit c319835 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #3550       +/-   ##
===========================================
- Coverage   63.14%   47.54%   -15.61%     
===========================================
  Files         308      309        +1     
  Lines       40836    40871       +35     
  Branches     5655     5381      -274     
===========================================
- Hits        25787    19432     -6355     
- Misses      14036    21321     +7285     
+ Partials     1013      118      -895     

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[effigies]

@yarikoptic Reran so you can check the output.