Shell2: 200× faster, persisted, multiplayer-native Shells

Replit introduced Shell2, a new shell backend that is approximately 200 times faster than the previous implementation, with persistent and multiplayer-native shells.

• The performance improvement comes from technical changes such as eliminating string-byte conversions, avoiding stdio, and reducing protocol introspection.
• The new system also enables shells to be shared and persisted across users in a multiplayer environment.