fix: set trailing positional arguments to active

This fixes an issue where existingfile et al would not correctly apply
to positional arguments with defaults.

kong_test.go

@@ -4,6 +4,8 @@ import (
 	"bytes"
 	"errors"
 	"fmt"
+	"os"
+	"path/filepath"
 	"strings"
 	"testing"
 
@@ -1984,3 +1986,16 @@ func TestEnumPtrOmittedNoDefault(t *testing.T) {
 	assert.NotZero(t, ctx)
 	assert.Zero(t, cli.X)
 }
+
+func TestTrailingPositionalActive(t *testing.T) {
+	var cli struct {
+		Arg string `arg:"" default:"." existingdir:"testdata"`
+	}
+	pwd, err := os.Getwd()
+	assert.NoError(t, err)
+	k, err := kong.New(&cli)
+	assert.NoError(t, err)
+	_, err = k.Parse([]string{})
+	assert.NoError(t, err)
+	assert.Equal(t, cli.Arg, filepath.Join(pwd, "testdata"))
+}