Include project language in project config

Currently, spacetime publish detects language by searching for Cargo.toml, and assuming the absence means we’re in a C# project. This is bad, because it means if you run spacetime publish in a directory that isn’t either a C# or Rust project, we’ll try to compile it as C#, which will fail with a bad error message.

One way to avoid this is to require that modules include a Spacetime config file (.spacetime.toml or spacetime.toml), and include the project language in that config file.

For good DX, this will require automatically creating a config file in spacetime init, and a good error message from spacetime publish if the config file is not found. The error message should describe how to create a config file, ideally including a snippet that the user can paste into their shell to create the config file.

Start by tracing the spacetime init and spacetime publish entry points and how they currently detect project language. Check how .spacetime.toml and spacetime.toml are handled, then define completion as config creation during init and a clear publish error with a shell-pastable creation snippet when config is missing.