installers: config to specify unpacking of other archived files

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.

The issue names only script installers and the install-path and unpack-all settings; no source file, test, or entry point is identified. Start by clarifying whether unpacking non-binaries is required and how updates should handle overlaid old files and directories. Done would be an agreed feature requirement and design, rather than an implementation patch.

Written by the indexing model from the issue text.