Add the ability to set a REVIEWERS_UNMODIFIED_EXIT_CODE, avoiding parallel action cancellation

[mcpengelly]

setting a REVIEWERS_UNMODIFIED_EXIT_CODE will allow consumers of this github action to set an exit status (my use case: exit code 0), this can be useful for preventing cancellation of parallel github actions if the assignee-to-reviewer action is skipped/not needed.

[abinoda]

@mcpengelly Thanks for this and the write up at #3! Any reason not to just hardcode a different exit code (not 78)?

[abinoda]

@mcpengelly Actually I understand better now... this makes sense and looks good!

[mcpengelly]

Mostly just for maintainability in the future (in case github changes anything), also can document it better this way in the README. I left the default behaviour of the action the same. but the user can specify their own if they'd like some other exit code. I referenced this other action which had the same functionality: https://github.com/jessfraz/branch-cleanup-action

I should note i havent actually had a chance to test it out yet because the response was so quick. Will be testing shortly.

Tested: works for my usecase. Thanks for the quick response!