This commit introduces the ability for packages to define an optional install script
within the `packages.conf` file. The `install.sh` script will now parse this
new field and execute the provided script after all associated package files
have been downloaded.
The `packages.conf` documentation has been updated to reflect the new format,
including the optional fifth field for the install script.
Performs a major refactoring of 'install.sh' to introduce non-interactive installation via command-line arguments (e.g., '--overwrite-config'). Enhances the interactive menu with detailed package information (display name, description, version, update status) and improves config file handling with diff previews. Updates 'packages.conf' format to support new package metadata and uses short, lowercase keys.
