Support forked stacks

[jeyj0]

GitButler stacks are currently linear. Support forked stacks, where multiple independent branches depend on one base branch:

main ── A ──┬── B
            └── C

This structure clearly indicates dependencies, and doesn't run into the issue of either B or C being blocked by the other during review.

(in theory allowing a branch to also depend on multiple other branches would be great too, but I fear we'll then enter the territory of too confusing UX)

My main use-case for this currently is fully AI-generated branches, so I'd actually be fully satisfied with CLI-only support for this (initially).