Extension mirroring

[GarboMuffin]

• clone and build TurboWarp/extensions, expose as eg. mirror.turbowarp.xyz/extensions/...
• add an API for runtime rewriting of actual loaded extension URL
• rewrite e.tw.o to the mirror at runtime
• stored URL still points to e.tw.o for compatibility

[Noob-dooN]

whould this make a locally downloaded version be able to use extensions offline? (my school network blocks "extensions.turbowarp.org")