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

common/hstrings/strings.go

@@ -99,3 +99,26 @@ var reCache = regexpCache{re: make(map[string]*regexp.Regexp)}
 func GetOrCompileRegexp(pattern string) (re *regexp.Regexp, err error) {
 	return reCache.getOrCompileRegexp(pattern)
 }
+
+// InSlice checks if a string is an element of a slice of strings
+// and returns a boolean value.
+func InSlice(arr []string, el string) bool {
+	for _, v := range arr {
+		if v == el {
+			return true
+		}
+	}
+	return false
+}
+
+// InSlicEqualFold checks if a string is an element of a slice of strings
+// and returns a boolean value.
+// It uses strings.EqualFold to compare.
+func InSlicEqualFold(arr []string, el string) bool {
+	for _, v := range arr {
+		if strings.EqualFold(v, el) {
+			return true
+		}
+	}
+	return false
+}