[Feature Request] Implement generation of extra players

[MrThatKid]

Is your feature request related to a problem? Please describe.

A good amount of modfiles made for NotITG recently take advantage of extra players. They are full players in everything but life, score and input (they can be autoplay, P1 or P2 input only), so they have a visible & functional judgment/combo (often hidden, but has been used in some cases), and can have their own set of modifiers applied to them, separate from P1 & P2 and the other extra players (this detail what's important for modfile makers). However, these extra players are always loaded on gameplay, which is probably not desirable.

Describe the solution you'd like

What I'd like to see is able to have some way to tell the game "I want X extra players to mess with for modding", and these would likely be considered as "virtual players", as they can't get input from anything other than P1/P2/AutoPlay (we'd probably need some way to set that).

As the issue title implies, these would only be brought in when the song/modder specifies it. The naming would probably use A-Z instead of numbers (PlayerPA? PlayerA?). (though I can't imagine ever going through the full alphabet for the extra players. nITG only has 6 of these extra players, for 8 total players as a reference) These would also probably start out as hidden, too.

Maybe this could be done with the song-specific Preferences.ini?