Gordon Kart 0.6: multiplayer-ish, simplification, and new areas

Greetings, fellow internauts!

After the PSX overhaul of… Christmas 2021 (oh my god this was so long ago), I am proud to present to you: Gordon Kart 0.6, now with (half-working) multiplayer!

For now, multiplayer was only successfully tested on localhost. Any Android <-> PC LAN tests were failures, but anyway! This is shipping, baby! Also it would have worked on gotm.io, had the platform not closed without any warning. Oh well!

Half-assed list of changes is the following:

• Port to Godot 3.6 (yes this is still on godot 3.x fight me if you want)
• Android release!
• Multiplayer (experimental)
• New lobby (billboard & customization are not implemented yet)
• New arena (not a race track)
• Removed the fuel & manual gearing systems
• New overhead camera, for those GTA 1 vibes or something
• New music
• New UI
• ???? (I didn't actually prepare a changelog)

Have fun!