Set up project as monorepo, rename project

Closes #30 Changes: - All references to "Commander" have been renamed to "Centurion", e.g. `CommanderServer` -> `CenturionServer` - Core and UI has been split into `@rbxts/centurion` and `@rbxts/centurion-ui` respectively - Interfaces now accept a `ClientAPI` argument, providing access to the client's registry, dispatcher and options - `CommanderInterface` -> `CenturionUI` - Package manager has been switched to Yarn due to better workspaces support with roblox-ts