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/config_test.go

@@ -106,7 +106,7 @@ func TestDecodeImageConfig(t *testing.T) {
 		if err != nil {
 			t.Fatal(err)
 		}
-		result, err := DecodeImageConfig(this.action, this.in, cfg, PNG)
+		result, err := DecodeImageConfig(this.action, strings.Fields(this.in), cfg, PNG)
 		if b, ok := this.expect.(bool); ok && !b {
 			if err == nil {
 				t.Errorf("[%d] parseImageConfig didn't return an expected error", i)