Make this available as a gh extension

[lakruzz]

Making it a gh extension allows us to assume, that the user has already configured gh correctly, and it gives the script a lot of gh built-in features to utilize.

gh extensisons are defined by the repo following a naming convention and git extensions requires the executable to follow a naming convention - so they could actually co exist in the same repo.

[lakruzz]

$ gh extension install lakruzz/gh-phlow
extension is not installable: missing executable

[lakruzz]

https://docs.github.com/en/github-cli/github-cli/creating-github-cli-extensions

[lakruzz]

e7efc9c did the trick!