Feature request: Default Reviewers

[luke-chisholm6]

I would like to add a concept of "default reviewers" to a repo config so that I do not need to specify the flag every single stack. I would also find it useful for my particular workflow if the flag added reviewers in addition to the default reviewers rather than overwrites them entirely, however this is subjective to my workflow so I would leave that up to the maintainers. I am willing to work on this feature and have a PR draft prepared.

#433