Duotone: using references in theme.json breaks site editor and don't work in front

[oandregal]

Related #49745 #49715

In theme.json styles can be referenced to an existing top-level style (docs).

The following example makes the image's duotone filter be linked to the top-level filter:

{
  "styles": {
    "filter": {
      "duotone": "var(--wp--preset--duotone--magenta-yellow)"
    },
    "blocks": {
      "core/image": {
        "filter": {
          "duotone": { "ref": "styles.filter.duotone" }
        }
      }
    }
  }
}

The expected result is that the images should have that filter applied.

Instead, what happens is that:

• front end: filter is not applied to the image block
• site editor: doesn't load
• post editor: the filter is applied to the image block
• reports an error at https://github.com/WordPress/gutenberg/blob/trunk/lib/class-wp-duotone-gutenberg.php#L136

[oandregal]

cc @ajlende @jeyip