Extend nupackage.toml to include Plugins?

[freepicheep]

This would help Quiver and other package managers know which version of Nu the plugin was compiled for. We would only need to expand the manifest spec with a package.type field that is either "module" or "plugin".

[package]
name = "nu_plugin_tui"
version = "0.1.3"
description = "A theoretical TUI editor for Nu structured data"
type = "plugin"
license = "MIT"
authors = ["Friedrich Stoltzfus"]
nu-version = "0.111.0"

Quiver isn't involved in the process of building plugins, so maybe this is unnecessary. We could also read the dependencies.nu-protocol from the plugin's Cargo.toml to determine the Nu version, so a nupackage.toml wouldn't be necessary for plugin repos.