Separate Triton into platform modules

[diogotcorreia]

Currently, the module structure is:

Triton
├── api
├── core
├── spigot-legacy
└── v1_13

The api module should stay (and be renamed to triton-api). The core module should be split into 4 separate modules: triton-core, triton-spigot, triton-bungeecord and triton-velocity, to ensure full separation of platforms and depedencies. The spigot-legacy and v1_13 can probably cease to exist, replacing them with reflection.

The resulting structure would be:

Triton
├── triton-api
├── triton-core
├── triton-spigot
├── triton-bungeecord
└── triton-velocity

Progress:

• [ ] Rename triton-api
• [ ] Rename triton-core
• [x] Extract triton-spigot
• [x] Extract triton-bungee
• [x] Extract triton-velocity
• [x] Ensure triton-core does not contain platform-specific code
• [ ] Remove spigot-legacy
• [ ] Remove v1_13