Check for depedencies #124

I think with the PRs it is likely that there is going to need to be some lock files needed. I seemed to have missed this. Got this error:

Dependencies lock file is not found in /home/runner/work/whisper-slides/whisper-slides. Supported file patterns: package-lock.json,npm-shrinkwrap.json,yarn.lock

Perhaps this needs to be called out more loudly in the documentation? Maybe there's a script that could be run to check that things are ready for the accessibility-scanner? Not sure.

Maybe just better error messages?