Allow storing greenzone in branches

I was talking to Memory on Discord, and she mentioned one of the deterrents for upgrading to a newer BizHawk is that branches in TAStudio no longer store greenzone and simply store a single savestate, making it extremely inconvenient to switch branches and have to rewatch a good bit of the TAS.

I would guess that change was made to save disk space/RAM, but it should be an option for the user to make that tradeoff if they're constantly switching branches and don't want to have to rewatch significant portions of the TAS just to load a branch. I myself have found this lack of savestates upon loading a branch to be slightly annoying.

I could see one arguing that this could cause desync problems, but I can't really see how that on its own would be the case. Savestates from branches aren't too different from any other savestates, and if there's a problem, it would seem more likely to be caused by savestates fundamentally having a problem rather than caused by the specifics of branch savestates.

Start by locating TAStudio's branch storage and loading paths, then review how branches currently retain a single savestate rather than greenzone data. Add a user-controlled option for retaining branch greenzone and verify that switching branches can restore the retained state without requiring rewatching.