all: apply modernisers

Signed-off-by: Ophestra <cat@gensokyo.uk>

command/parse.go

@@ -91,8 +91,8 @@ func (n *node) MustParse(arguments []string, handleError func(error)) {
 	case ErrEmptyTree:
 		os.Exit(1)
 	default:
-		var flagError FlagError
-		if !errors.As(err, &flagError) { // returned by HandlerFunc
+		flagError, ok := errors.AsType[FlagError](err)
+		if !ok { // returned by HandlerFunc
 			handleError(err)
 			os.Exit(1)
 		}