Allow to configure parsers in packwerk (continued)

[richardmarbach]

What are you trying to accomplish?

Since no further development has taken place on #243, I hope it's ok that I've taken the liberty to implement the requested changes from #243 (comment)

What approach did you choose and why?

What should reviewers focus on?

This is the first time I've used minitest. Would love some feedback on if there's a better way to write the tests.

Type of Change

• Bugfix
• New feature
• Non-breaking change (a change that doesn't alter functionality - i.e., code refactor, configs, etc.)

Additional Release Notes

• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• It is safe to rollback this change.