Add dependency sync entries

You can now declare project dependencies as [[deps]] entries in fabrik.toml and sync them with fabrik deps sync [name]. This gives build targets one dependency list, where local targets are referenced as strings and external ecosystem dependencies (Rust, Swift, Go, and Elixir) are referenced as inline tables like { cargo = “serde” }. For Rust, external dependencies are lowered into generated vendor// targets with corresponding declarations under vendor//fabrik.toml. The existing fabrik vendor command remains as a hidden alias for compatibility.