Handle workflow run

[calumy]

This PR updates the action to handle the assignment of PRs and issues to users when triggered from a workflow_run.

It also fixes the action to allow authors to be assigned to PRs that they have created but does not allow them to be assigned as a reviewer.

[codecov[bot]]

Codecov Report

Merging #65 (b69672c) into v1 (fff0a74) will decrease coverage by 6.20%. The diff coverage is n/a.

@@            Coverage Diff             @@
##               v1      #65      +/-   ##
==========================================
- Coverage   91.02%   84.82%   -6.21%     
==========================================
  Files           1        1              
  Lines          78      112      +34     
  Branches       23       36      +13     
==========================================
+ Hits           71       95      +24     
- Misses          7       17      +10     

Impacted Files	Coverage Δ
src/action.js	84.82% <0.00%> (-6.21%)	:arrow_down:

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[calumy]

Codecov failing for a similar reason to #64.

[pozil]

Thanks a lot for these contributions @CalumY, much appreciated.