Addons Menu backport to vanilla

If y'alls feel like reviewing this for .22, here it is! Thanks to @toaster for the encouragement. Merged nearly verbatim (tweaked as necessary) from https://git.magicalgirl.moe/KartKrew/Kart-Public/commit/8bc1a217fe9785a84ed783eb4317741bc7f4e696

Loading WADs, PK3, and scripts work. Search works. Configurable folder path works. 32/64-bit work. Didn't touch much else.

I'm not familiar with the netcode-touching part of this. You'll have to verify whether it's safe.

Any other things I missed, sorry about that. I didn't read the source too carefully...

Resource WAD

2122-menu.wad