feat: introduce clientBundle.scan

[antfu]

This PR introduces the clientBundle.scan option, which will do a static scan for all components/markdowns and include them in the client bundle. This would resolve #34

Note

For v1.5.x, this option is NOT enabled by default for experiments. We might consider to enable it in the future version.
To enable it, set:

export default defineNuxtConfig({
  modules: [
    '@nuxt/icon'
  ],
  icon: {
    clientBundle: {
      scan: true, // <!--
    },
  },
})

Also introduced the clientBundle.sizeLimitKb option to guard and avoid resulting oversized bundle.