Expand savestates so you can load savestates other than the latest one, similar to LoTAS

[ScribbleTAS]

Created from card: Recording/playback starting from the main menu

Requirements

A savestateIndex that says what savestate to create next. When you press the hotkey J a savestate should be created on the next index, since we have none, this would be one. Then the savestateIndex will be 1.
If we savestate again, the created savestate should be 2, and the savestateindex should be 2 as well. If we load the savestate with the hotkey, the savestate at the savestate index should be loaded, in this case it's the latest one.
So far, this should sound familiar to the current system in LoTAS.
Now, let's say we implement a command with /savestate load 1. Now we load the savestate at the index 1 and set the savestateIndex to 1 as well. Now if we savestate, we overwrite the second savestate. This is currently not possible with TASmod.
We also save the current savestateIndex to the currentWorld file.

[ScribbleTAS]

Reminder for me to implement something that can load savestates while the integrated server is closed... this obviously works only in singleplayer