Release v3.21.1

Fixed

• The binding module imports each generated proxy from the slice folder that declares it, instead of from the project root where no proxy is written. Every import previously pointed at a file that does not exist. (#100)
• The generated frontend declares the types its own bundler configuration needs, so it type-checks and its build script runs. (#100)

Found by generating an application and building it: the backend builds with no warnings, the frontend installs, type-checks and bundles, and the bundle contains the command form and the Arc binding boundary the composed screen depends on.