Add arm64 macOS (Apple Silicon) build

Warum ist der aktuelle Stand nicht akzeptabel?

The current macOS binaries produced by GitHub Releases don't run on ARM Macs as they bundle native x86_64 JavaFX libraries. Strangely running java under Rosetta doesn't work either, but since Java is largely cross-platform we should probably avoid emulation entirely.

Was wäre dein Lösungsansatz?

Provide a build for the macos-aarch64 (aka. osx-aarch64) platform as described in the upstream Gradle-JFX plugin.

While having GitHub-provisioned ARM runners would make this easy (since we could just add a new OS to the matrix), these aren't available yet. Still, making cross-builds should be doable since the JVM code is cross-platform anyway, so we would only have to ensure that Gradle bundles the native libraries for the specified target architecture. Still, that might require some fiddling, as evidenced by this thread:

• https://github.com/openjfx/javafx-gradle-plugin/issues/99

Locate the Gradle configuration that produces the GitHub Releases macOS binaries and compare it with the upstream Gradle-JFX plugin guidance for cross-platform projects. Check the linked plugin issue for cross-build constraints; done means a macos-aarch64/osx-aarch64 release is produced with native ARM libraries and runs without Rosetta on Apple Silicon.