feat: Add a new Github Actions Reporter

[Saadnajmi]

Github Actions largely is similar to Azure Pipelines in that you can add special syntax to your logs to group them and stuff... but the syntax is different: https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-commands#grouping-log-lines

I asked Claude to make a new GroupedReporter base class, which AdoReporter and a new GithubActionsReporter derive from.

I also updated createReporter to detect if we're in Azure Pipelines or Github Actions, and change the default reporter to the CI-environment specific one.