Hotkeys for direct save/load state slots

First and foremost consider this:

• Only RetroArch bugs should be filed here. Not core bugs or game bugs
• This is not a forum or a help section, this is strictly developer oriented

Description

I would like to see a set of hotkeys to directly save or load into states 1-6, similar to other emulators such as Dolphin. Functionally, this would leverage the existing slot system by setting the slot number directly then saving/loading into that slot.

For example, if I'm currently on slot 3 and I press the hotkey to save into state 5, it'll set the state number to 5 then directly save into state 5. There can be another option to toggle these hotkeys.

I'm open to working on a PR for this.

Some more context about my specific use-case: I run a Twitch Plays channel with inputs that are restricted by user level (Moderator, Admin, etc). On emulators such as Dolphin, I can reserve savestate 1 to Moderators and up, but on RetroArch there is only one savestate input due to the slot system, so this is not possible. Adding these hotkeys would enable me to restrict states based on level in RetroArch.

Start by tracing RetroArch's existing state-slot save/load handling and hotkey configuration. Check how the current slot number is set, then determine how direct save/load actions for slots 1-6 and an option to toggle them should fit the existing behavior. Done means each requested hotkey directly saves or loads its assigned slot without requiring a separate slot change.