Write ExUnit formatter that will use Workflow Commands

[hauleth]

This is IMHO better approach than producing the SARIF report. Mostly because SARIF is meant for reporting of static analysis of the code and tests aren't such thing. Using Workflow Commands should be "more natural" and should be easier to implement properly.

Ref #11

cc @axelson

[axelson]

That seems like a good approach

Seems like outputting a warning message is what we want:
https://docs.github.com/en/actions/learn-github-actions/workflow-commands-for-github-actions#setting-a-warning-message

[hauleth]

I was thinking more about error level for test failures and info for skipped tests.