New Sandbox Loader

[TheCodedOne]

Sandbox loader has never had a fully complete rethink since we started, it maintains a lot of ancient code from the start of the project.
With Forge support being worked on now is a good time to not only rewrite the Fabric implementation but unify both implementations.

The goal of 2.0 should be to improve code maintainability across both variants but also fix previously broken or unfinished features & add new ones.

• Server -> Client addon syncing
• Server -> Client config syncing
• Password protected servers
• In-game mod management
• Clientside-only addons
• OBJ Model Support

Unimportant features:

• Discord Integration
• Chromium Integration
• Velocity Integration

New features are welcome for discussion, new APIs should follow the API design process at https://github.com/SandboxPowered/SandboxDesign