Plugins: allow using other plugins

[Florian Lefebvre]

A plugin may define a dependency on another plugin, an idea of API:

import { otherPlugin } from "./other-plugin.js"

export const dependentPlugin = definePlugin({
	name: "foo",
	hook: "astro:config:setup",
	dependsOn: [otherPlugin],
	implementation: ({ otherPluginName }) => () => {}
})