manifest plugin: handle dynamic imports

[0xb4lint]

I'm using vite with Vue 3 TS and lazy loading routes.

Previously (with vite 1) I was able to generate preload meta tags using shouldPreload() function, but now I'm trying to migrate this functionality to a custom plugin using the manifest.json file.

Currently the manifest plugin generates the following output:

{
  "index.js": {
    "file": "assets/index.47f0281f.js",
    "imports": []
  }
}

With this little PR the manifest.json will contain all the dynamic imports in the following way, allowing a much wider area of use:

{
  "index.js": {
    "file": "assets/index.47f0281f.js",
    "imports": [],
    "dynamicImports": [
      "assets/About.e27cd12c.js"
    ]
  },
  "About.js": {
    "file": "assets/About.e27cd12c.js",
    "imports": [
       "assets/index.47f0281f.js",
     ],
    "dynamicImports": []
  }
}