fix(dev): handle hard restart

[pi0]

resolves #135

hard restart support were missing in initial new dev server impl.

This PR also fixes small relevant issues:

• avoid mutating process.argv (for subsequent reloads)
• register (main) graceful shutdown hooks once
• avoid exiting main process when child restarts with code 0 (internally by parent)

Playground: (WSS is buggy, need to rename playground/modules dir)

export default defineNuxtConfig({
  hooks: {
    ready(nuxt) {
      setTimeout(() => {
        console.log('sending hard restart...')
        nuxt.callHook('restart', { hard: true })
      }, 1000)
    },
  },
})