Return secrets as outputs instead of values (no dist)

[Olfi01]

Issue #14: Environment variables vs outputs

Description of changes:
Secrets are now returned as action outputs. This results in the following improvements:

• User side:
  • Less concern needed over which steps to separate in a different job to avoid access to environment variables
  • Full control over where the returned secrets are used
• Codebase side:
  • No more need to clean up environment variable secrets, as the GitHub Actions toolkit will handle the outputs.

README.md, action.yml and code comments have been updated accordingly. Tests are passing.

If you prefer having the compiled files included in the PR, I have created a parallel PR (#36) containing the code changes and the built dist files.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[Olfi01]

Since I seem to consistently fail keeping my build files out, I'll close this PR. A re-build of the TS code is still possible on the other PR.