App Bundle for MacOS

The current system causes issues with MacOS Gatekeeper because you need to approve both your app's executable and the python executable in Gatekeeper, which can be tricky to do. Having one .app file solves this and is much better user experience for consumer apps, because the user doesn't have to dig through the bin/ directory to find the right file. PyInstaller --onefile mode solves this but that creates a startup delay of having to unpack everything into a temporary directory every time it starts.

On MacOS, .app files are actually directories, so it's pretty straightforward to bundle the py-app-standalone outputs into an app which solves all of these problems. I had Claude Code write a script to do this and it works well so far, including doing automated builds with Github Actions, you can see it [here](https://gist.github.com/ahalekelly/c270b7a3584c398af74b103065e1eb5e). It would be great to have this functionality built into py-app-standalone.

A single portable executable file for Windows would be great too but I don't have ideas for how to do that.