Add images.Process filter

This allows for constructs like:

```
{{ $filters := slice (images.GaussianBlur 8) (images.Grayscale) (images.Process "jpg q30 resize 200x") }}
{{ $img = $img | images.Filter $filters }}
```

Note that the `action` option in `images.Process` is optional (`resize` in the example above), so you can use the above to just set the target format, e.g.:

```
{{ $filters := slice (images.GaussianBlur 8) (images.Grayscale) (images.Process "jpg") }}
{{ $img = $img | images.Filter $filters }}
```

Fixes #8439

resources/images/process.go

@@ -0,0 +1,43 @@
+// Copyright 2023 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package images
+
+import (
+	"image"
+	"image/draw"
+
+	"github.com/disintegration/gift"
+)
+
+var _ ImageProcessSpecProvider = (*processFilter)(nil)
+
+type ImageProcessSpecProvider interface {
+	ImageProcessSpec() string
+}
+
+type processFilter struct {
+	spec string
+}
+
+func (f processFilter) Draw(dst draw.Image, src image.Image, options *gift.Options) {
+	panic("not supported")
+}
+
+func (f processFilter) Bounds(srcBounds image.Rectangle) image.Rectangle {
+	panic("not supported")
+}
+
+func (f processFilter) ImageProcessSpec() string {
+	return f.spec
+}