Add an output that specifies the assignment happened

[ManuelRauber]

Hi,

it would be nice if the action has an output that specifies if an assignment has actually happend to make further processing based on that decision.

[bdougie]

I am intrigued, are you able to provide an example of the additional context @ManuelRauber

[koppor]

For example, there is an action changed the state in a GitHub project: https://github.com/m7kvqbe1/github-action-move-issues/

In case a contributor was assigned, the state should change from "Free to take" to "Assigned".

Example implementation: https://github.com/JabRef/jabref/blob/25635de59496e2765ccfab19c07e91b6c48067a4/.github/workflows/on-labeled-issue.yml#L49

The issue should not change the state if no assignment happened.