swift-string-counter-web でやると別のエラーが出ますね．(Linux aarch64 環境なので，macOS だと結果は異なるかもしれませんが)

wasm-ld: error: /home/kebo/.swiftpm/swift-sdks/swift-wasm-DEVELOPMENT-SNAPSHOT-2024-05-02-a-wasm32-unknown-wasi.artifactbundle/DEVELOPMENT-SNAPSHOT-2024-05-02-a-wasm32-unknown-wasi/wasm32-unknown-wasi/swift.xctoolchain/usr/lib/swift_static/wasi/wasm32/swiftrt.o: undefined symbol: swift_addNewDSOImage wasm-ld: error: /home/kebo/swift-string-counter-web/.build/wasm32-unknown-wasi/debug/_CJavaScriptKit.build/_CJavaScriptKit.c.o: undefined symbol: _call_host_function_impl wasm-ld: error: /home/kebo/swift-string-counter-web/.build/wasm32-unknown-wasi/debug/_CJavaScriptKit.build/_CJavaScriptKit.c.o: undefined symbol: _free_host_function_impl wasm-ld: error: /home/kebo/swift-string-counter-web/.build/wasm32-unknown-wasi/debug/_CJavaScriptKit.build/_CJavaScriptKit.c.o: undefined symbol: _library_features clang: error: linker command failed with exit code 1 (use -v to see invocation)

(edited)