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/errorResource.go

@@ -128,7 +128,7 @@ func (e *errorResource) Exif() *exif.ExifInfo {
 	panic(e.ResourceError)
 }
 
-func (e *errorResource) Colors() ([]string, error) {
+func (e *errorResource) Colors() ([]images.Color, error) {
 	panic(e.ResourceError)
 }