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/errorResource.go

@@ -100,6 +100,10 @@ func (e *errorResource) Width() int {
 	panic(e.ResourceError)
 }
 
+func (e *errorResource) Process(spec string) (images.ImageResource, error) {
+	panic(e.ResourceError)
+}
+
 func (e *errorResource) Crop(spec string) (images.ImageResource, error) {
 	panic(e.ResourceError)
 }