Add support for concurrent steps execution

Act version

0.2.89

Feature description

GitHub Actions now supports running steps concurrently ! It would be very useful to add support in act. Currently act fails with a bunch of "Unknown Property" errors

Announcement on GitHub blog post Syntax documentation (Beware, at the time of creation of this issue the documentation for the new syntax is only available in english !)

Here is an example of a workflow using parallel steps : https://github.com/nicopico-dev/petit-boutiste/actions/runs/28329958323/job/83926316443