[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 ....
```

首先追蹤現有的 `gh stack up` 和 `gh pr edit` 入口，以及如何取得 stack 的 PR 清單。比較提出的兩種命令形式，然後將完成定義為透過一次呼叫將要求的編輯套用至 stack 中的每個 PR，同時包括對所選介面的涵蓋。

由索引模型根據 Issue 內容生成。