Pathogen repo linter

[joverlee521]

(Jotting down some discussion I had with @tsibley in our 1:1)

Currently the goal is to keep the zika and mpox repos up-to-date has the "gold standard" repos, so we can just do changes piecemeal. When making a PR in the pathogen-repo-guide, make the corresponding changes in zika and mpox as examples of how the changes will affect in-use workflows.

In the future, it'd be nice to have a linter to list clear steps on how to update pathogen repos to adhere to the pathogen-repo-guide. This can start out as an internal tool for Nextstrain core pathogens, but we can potentially expand it to a program that can be run by external users to update their pathogen repos.