Feature Request: Add `gh stack split` to split the current branch at a chosen commit

## Problem
A branch often grows into two or more reviewable concerns, splitting these after the fact is tedious
Today it requires a manual operation: insert 1 PR above, cherry pick commits from PR1 to new PR2, rebase
If more than 1 split is required, it's even more error prone manipulations

## Proposal

`gh stack split [] [--at ]`

- Current branch keeps commits up to ``
- `` and commits above move to ``, inserted **directly above** the current branch in the stack (if some PR are already stacked above current PR)
- Branches already above are rebased onto the new layer; existing PRs keep their identity, the current PR's base is unchanged

Ideally, interactive mode lists the current branch's commits, user picks the split point and types the new branch name: this could happen in the `gh stack modify` view

從現有的 gh stack 命令入口點和 gh stack modify 檢視開始，因為提案建議在那裡整合互動式拆分選擇。定義拆分對目前分支、已經位於其上方的分支以及 pull request 身分的行為，然後驗證 gh stack split 支援指定的分支和提交選項，並保留所述的 stack 關係。

由索引模型根據 Issue 內容生成。