installers: config to specify unpacking of other archived files

[Gankra]

Only applies to script installers (shell, powershell).

When install-path = "CARGO_HOME" was the only possible setting it didn't make sense to unpack anything but binaries, but other options like install-path = "~/.my-app" would be very reasonable to allow more verbose unpacking of things like READMEs/LICENSES.

The most simplistic design would just be unpack-all = true, but there's questions about overlaying old files/dirs (for updates) that might be worth being able to elaborate on.

Need someone to chime in with this as a requirement before I try to design the feature.