Writes the LibrarySpec of a library module, the library.json other
clients (a React app, a mobile app) build their library from.
mix openui.gen.spec MyApp.Library --out assets/library.jsonWith --check nothing is written. The task fails when the new spec would
break programs stored with the old one (a prop moved or removed, a new
required prop without a default), when the file is out of date, or when it
is not a valid LibrarySpec. Run it in CI so the file never drifts from the
module.