[Feature request] add `each` mode to batch edit PRs in a stack

[andreakeesys]

If I want to request the review of each PR to a list of collaborators, I need to loop gh stack up && gh pr edit --add-reviewer ... for each PR in the stack, while with:

$ gh stack each edit --add-reviewer ....

the edit --add-reviewer command could be applied to each PR of the stack automatically with a single request.

If we would like to limit the each scope only to the pr edit action, then we could simplify it as:

$ gh stack each --add-reviewer ....