installer: macOS pkg

[davidkna]

I think it would be nice to support creating pkg installers for macOS. Compared to dmg/app this could be more suitable for CLI applications (#24). For example, starship publishes pkg installers.

[indygreg]

FYI I have a pure Rust pkg file format implementation at https://crates.io/crates/apple-flat-package. Unfortunately, it doesn't currently support writing because my Bom file implementation (https://github.com/indygreg/apple-platform-rs/tree/main/apple-bom) doesn't yet support writing. It's an unknown amount of work to finish implementing writing support.

I just wanted to let you know that generating pkg files without using Apple tooling is just outside the realm of possibility.

(The linked repository also has a pure Rust implementation of Apple code signing and notarization so you can sign + notarize from Linux in case that's interesting to you.)