Danger check for changes to `packwerk.yml`

[alexevanczuk]

Gusto has an internal danger plugin to watch for changes to packwerk.yml. We use this to just notify of these PRs because this is a sensitive file that shouldn't be changed day to day.

If there is interest, we could port this over to danger-packwerk.

[schoblaska]

This seems like potentially a good idea! Leaving this issue open for now to track, though it is probably low priority for us.