webmanifest - Add support for icons in file_handlers

[grimsteel]

↪️ Pull Request

This adds support for bundling images referenced in file handler icons in webmanifests (related to #8660)

💻 Examples

{
  "name": "Example",
  "icons": [{
    "src": "images/icon48.png",
    "sizes": "48x48",
    "type": "image/png"
  }],
  "file_handlers" : [
    {
      "icons": [{
        "src": "images/file-handler-icon.png",
        "sizes": "48x48",
        "type": "image/png"
      }]
    }
  ]
}

🚨 Test instructions

Covered by webmanifest integration tests

✔️ PR Todo

• Added/updated unit tests for this change
• Filled out test instructions (In case there aren't any unit tests)
• Included links to related issues/PRs

[mischnic]

Thank you!