Support creating installers instead of executables

[tusharsadhwani]

Currently packaged creates binaries that simply extract into a temporary directory and run the program. But for applications that want to store state relative to the package, or add a desktop shortcut etc., this is not the best solution.

So we should add a --create-installer flag that would make the binary install the app in a directory and add it to PATH instead.

Should correspond to to build-type = "installer" vs. "executable" in the packaged.toml config file.