commands: Make the --poll flag a duration

So you can do:

```
hugo server --poll 700ms
```

See #8720

hugolib/site.go

@@ -29,6 +29,8 @@ import (
 	"strings"
 	"time"
 
+	"github.com/gohugoio/hugo/common/types"
+
 	"github.com/gohugoio/hugo/common/paths"
 
 	"github.com/gohugoio/hugo/common/constants"
@@ -523,13 +525,9 @@ But this also means that your site configuration may not do what you expect. If
 	timeout := 30 * time.Second
 	if cfg.Language.IsSet("timeout") {
 		v := cfg.Language.Get("timeout")
-		if n := cast.ToInt(v); n > 0 {
-			timeout = time.Duration(n) * time.Millisecond
-		} else {
-			d, err := time.ParseDuration(cast.ToString(v))
-			if err == nil {
-				timeout = d
-			}
+		d, err := types.ToDurationE(v)
+		if err == nil {
+			timeout = d
 		}
 	}