Add Luminance to Color

To sort an image's colors from darkest to lightest, you can then do:

```handlebars
{{ {{ $colorsByLuminance := sort $image.Colors "Luminance" }}
```

This uses the formula defined here: https://www.w3.org/TR/WCAG21/#dfn-relative-luminance

Fixes #10450

resources/images/image_resource.go

@@ -63,7 +63,7 @@ type ImageResourceOps interface {
 
 	// Colors returns a slice of the most dominant colors in an image
 	// using a simple histogram method.
-	Colors() ([]string, error)
+	Colors() ([]Color, error)
 
 	// For internal use.
 	DecodeImage() (image.Image, error)