Add support for `world` in MV3

https://groups.google.com/a/chromium.org/g/chromium-extensions/c/_zKyp9XvIzY/m/a5fV4MTcAgAJ

This can now appear in the manifest:
```js
 "content_scripts": [{
    "js": ["content.js"],
    "matches": ["<all_urls>"],
    "run_at": "document_start"
  }, {
    "world": "MAIN",
    "js": ["page.js"],
    "matches": ["<all_urls>"],
    "run_at": "document_start"
  }],
```