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

Comienza con el punto de entrada existente del comando gh stack y la vista gh stack modify, ya que la propuesta sugiere integrar allí la selección interactiva de división. Define el comportamiento de la división para la rama actual, las ramas que ya están por encima de ella y la identidad del pull request; después, verifica que gh stack split admita las opciones de rama y commit indicadas y conserve las relaciones de stack especificadas.

Escrito por el modelo de indexación a partir del texto del issue.