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

[brian-xu-vlt]

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 [<new-branch>] [--at <commit>]

• Current branch keeps commits up to <commit>
• <commit> and commits above move to <new-branch>, 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