CLI: Search upwards for `Cargo.toml` to detect `--project-path`

[bfops]

Several CLI subcommands (publish, build, generate, possibly others) attempt to determine whether the project is Rust or C# by looking for a Cargo.toml file in the current directory (or the --project-path if passed), and assume C# if the Cargo.toml is not found. This is a bad system and should be changed, but at the very least, they should search up for the Cargo.toml so that it's possible to run the commands from a subdirectory of a Rust project (e.g. in src/).

[bfops]

We improved the language inference behavior here: https://github.com/clockworklabs/SpacetimeDB/pull/2218

We still only look in the current directory, though.