Return a warning for the service_worker property

[regseb]

Describe the problem and steps to reproduce it:

{
  "manifest_version": 3,
  "name": "Test case",
  "version": "1.0.0",
  "background": {
    "service_worker": "background.js",
    "scripts": ["background.js"]
  }
}

What happened?

Validation Summary:

errors 1
notices 0
warnings 0

ERRORS:

Code	Message	Description	File	Line	Column
MANIFEST_FIELD_UNSUPPORTED	"/background/service_worker" is not supported.	"/background/service_worker" is not supported.	manifest.json

What did you expect to happen?

• If the background property has a scripts and a service_worker: return no error (or only a warning) for the service_worker property.
• If the background property has only a service_worker: continue to return an error for the service_worker property.

Anything else we should know?

• Proposal: declaring background scripts in a neutral way w3c/webextensions#282
• Background page fails to load if background.service_worker and background.scripts are both present (Bugzilla)
• background / Browser support (MDN)

┆Issue is synchronized with this Jira Task