Add rule to detect GitHub Actions using default permissions

[stacklokbot]

I built & presented this rule in a tutorial at CloudNativeSecurityCon: https://cloudnativesecurityconna24.sched.com/event/8470542653b26dbcab395ccc294b81b3

[jhrozek]

@evankanderson thanks for the PR. I think you opened the PR from the wrong account by mistake - do you want to fix it?
Also, we have a workflow that fails if there's no tests for a new ruletype. I can write one (I even think it's better if someone else than the author writes the test..), do I read the rego code correctly that all that would be needed would be a workflow that doesn't define permissions explicitly?

[jhrozek]

tested manually, works fine.

[evankanderson]

Replaced with #134