Add $image.Process

Which supports all the existing actions: resize, crop, fit, fill.

But it also allows plain format conversions:

```
{{ $img = $img.Process "webp" }}
```

Which will be a simple re-encoding of the source image.

Fixes #11483

resources/images/image_resource.go

@@ -33,6 +33,9 @@ type ImageResourceOps interface {
 	// Width returns the width of the Image.
 	Width() int
 
+	// Process applies the given image processing options to the image.
+	Process(spec string) (ImageResource, error)
+
 	// Crop an image to match the given dimensions without resizing.
 	// You must provide both width and height.
 	// Use the anchor option to change the crop box anchor point.