Support setting `src` prefix for manifest generation

[typed-sigterm]

I'm using AssetPack + Pixi.js + Nuxt latest. I have entry: './app/resources', output: './public/packed', the assets are served on http://localhost:8508/packed/**

But AseetPack generated http://localhost:8508/packed/manifest.json is like:

{
  "bundles": [
    {
      "name": "default",
      "assets": [
        {
          "alias": [
            "cards"
          ],
          "src": [
            "cards-0.webp.json"
          ],
          "data": {
            "tags": {
              "tps": true
            }
          }
        }
      ]
    }
  ]
}

So when I'm at http://localhost:8508/rooms/1 Pixi.js always tries to fetch http://localhost:8508/rooms/1/cards-0.webp.json, not http://localhost:8508/packed/cards-0.webp.json. If there's a option to specify the prefix of src, the problem will be solved by setting it to /packed/.