🌐 Add links to fixtures

[fxedel]

With Schema 9.0.0, is it possible to add multiple links of specified types to fixtures, like so:

{
  // ...
  "links": {
    "manual": [
      "https://example.com/downloads/some-fixture-manual.pdf"
    ],
    "productPage": [
      "https://example.com/products/some-fixture"
    ],
    "video": [
      "https://www.youtube.com/watch?v=dQw4w9WgXcQ"
    ],
  },
  // ...
}

Having at least one manual, one product page and one video per fixture would be very helpful. Small pull requests to only add a single link are appreciated as well.