Liatir API packages

Node plugins use @liatir/api Node module as typed API entry point, based on a shared define_plugin contract;
Python and WASM plugins use the same define_plugin contract, but not as a package: the CLI scaffolds a single managed module (src/liatir.py / src/liatir.rs) into the project and liatir build keeps it in sync — there is nothing to install or update separately.