Add light-dark() image values feature (web-platform-dx#3974)

Issue: web-platform-dx#3962

Author: ddbeck

features/light-dark-image.yml

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+name: light-dark() image values
+description: The `light-dark()` CSS function accepts, in addition to colors, two `<image>` values, such as a gradient or URL, and uses one depending on the current color scheme.
+spec: https://drafts.csswg.org/css-color-5/#typedef-light-dark-image
+compat_features:
+  - css.types.color.light-dark.image_value
+# Future merge candidate with `light-dark`

features/light-dark-image.yml.dist

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+# Generated from: light-dark-image.yml
+# Do not edit this file by hand. Edit the source file instead!
+
+status:
+  baseline: false
+  support: {}
+compat_features:
+  - css.types.color.light-dark.image_value