Command is validated before flags

Command is validated before its flags. I expect command validation to operate on already-validated flags. Here is a simple demonstration: https://go.dev/play/p/fk5EOSl7MuN

```go
type CLI struct {
Command Command `cmd:""`
}

type Command struct {
Flag Flag
}

func (Command) Validate() error {
fmt.Println("validate command")
return nil
}

type Flag string

func (Flag) Validate() error {
fmt.Println("validate flag")
return nil
}

func main() {
kong.Must(&CLI{}).Parse([]string{"command", "--flag", "value"})
// validate command
// validate flag
}

```