Idea: Horizontal scopes

Operations in git-spice have scopes: branch, upstack, downstack, and stack.
These can be visualized as vertical scopes.

This works quite well when the branches are stacked and their changes are inter-dependent.
A slightly more complicated scenario that I run into sometimes is this:
N > 1 branches that are not dependent on each other, but are closely related—to the same theme, for example.
E.g., a migration in a large repository separated into N independently mergeable changes all against the main branch.

```mermaid
flowchart BT
main --> Folder1 & Folder2 & Dots & Folder3
Dots["..."]
```

In such a set up, it would be quite useful to be able to perform at least submit and restack on all N branches together.

I haven't fully thought out the UX here, but I'm imagining a means of tagging branches with tags (or "theme"?), and perhaps being able to do `gs theme submit` or similar. Still need to think of naming and terminology here.