test that flutter_rust_bridge works

flutter_app/rust_builder/cargokit/build_tool/README.md

@@ -0,0 +1,5 @@
+/// This is copied from Cargokit (which is the official way to use it currently)
+/// Details: https://fzyzcjy.github.io/flutter_rust_bridge/manual/integrate/builtin
+
+A sample command-line application with an entrypoint in `bin/`, library code
+in `lib/`, and example unit test in `test/`.