(Config) Attempt to create SaveState Folder on save if it doesn't exist

Description

Setting a custom Savestate folder is not working.

Steps to reproduce the bug

1. Load a core
2. Settings > Directory > Savestate
3. Change the Savestate folder
4. Main Menu > Configuration File > Save Current Configuration
5. Exit RetroArch
6. Open "retroarch.cfg" and check "savestate_directory" you will see that it's still set to the default
   savestate_directory = ":\states"

I have also tried manually changing the "savestate_directory" value in "retroarch.cfg" but RetroArch refuses to apply the changes. When I check the "Savestate" folder in the Settings > Directory, it always shows the path to my ":\states" folder.

Setting a custom "Saves" folder works just fine though, only "Savestates" is not working.

Version/Commit

This is happening in RetroArch v1.7.7 all the way up thru the latest v1.8.2 builds.

Environment information

This only appears to be happening in the Windows version. I tested this on my Nintendo Switch and I wasn't having this issue. I'm thinking it probably has something to do with the Windows folder structure and using backslashes instead of Linux structure using forward slashes.

Reproduce the issue on the Windows version using the listed Settings > Directory > Savestate and configuration-save steps, then inspect the savestate_directory handling. Compare it with the working Saves directory behavior. Done means a custom savestate path persists in retroarch.cfg and is applied after restarting RetroArch, including when the folder does not yet exist.