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

Comece pelo ponto de entrada existente do comando gh stack e pela view gh stack modify, pois a proposta sugere integrar ali a seleção interativa de divisão. Defina o comportamento da divisão para o branch atual, para os branches que já estão acima dele e para a identidade do pull request; em seguida, verifique se gh stack split oferece suporte às opções de branch e commit indicadas e preserva as relações de stack especificadas.

Escrita pelo modelo de indexação a partir do texto da issue.