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

Inizia dall’entry point esistente del comando gh stack e dalla vista gh stack modify, poiché la proposta suggerisce di integrare lì la selezione interattiva della divisione. Definisci il comportamento della divisione per il branch corrente, per i branch già presenti sopra di esso e per l’identità della pull request, quindi verifica che gh stack split supporti le opzioni di branch e commit indicate e preservi le relazioni di stack specificate.

Scritto dal modello di indicizzazione a partire dal testo della issue.