gh stack checkout only works for formal stacks

[mdaniel]

Summary

It seems GitHub, and by consequent the gh and gh stack CLI behaviors, have two different mental models for what a "PR" is: one must now use gh pr checkout 12345 if it is a normal PR and gh stack checkout 12345 if it is part of a GH Stack

Then gh stack checkout does not simply fall back to the gh pr checkout behavior if the given PR number is not part of a formal stack

That places the burden upon any automation, or reviewers, to have an if statement which either looks up the condition of the PR or does an actual if ! gh stack checkout $num; then gh pr checkout $num; fi work-around

Expected Behavior

$ gh stack checkout 12345
✗ PR #12345 is not part of a stack on GitHub, using $(gh pr checkout) instead
remote: Enumerating objects: 11, done.
...etc