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

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:
```sh
$ 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:
```sh
$ gh stack each --add-reviewer ....
```

Start by tracing the existing `gh stack up` and `gh pr edit` entry points and how a stack's PR list is obtained. Compare the two proposed command forms, then define completion as applying the requested edit to every PR in the stack with one invocation, including coverage for the chosen interface.

Written by the indexing model from the issue text.