Enums() type doesn't tab complete

[stroopc-stripe]

When using an Enums() flag, I noticed that it doesn't tab complete like an Enum() does. This was unexpected since the only difference is that an Enums allows multiple values from the set to be provided

Steps to reproduce

Save as main.go

package main

import (
	"github.com/alecthomas/kingpin/v2"
)

var (
	noComplete = kingpin.Flag("does-not-complete", "this flag does not tab complete").Enums("foo", "bar")
	complete   = kingpin.Flag("completes", "this flag will tab complete").Enum("foo", "bar")
)

func main() {
	kingpin.Parse()
}

Then:

go mod init test-complete
go mod tidy
go build -o test-complete main.go
# use --completion-script-zsh if you're using zsh
eval "$(./test-complete --completion-script-bash)" 

# "foo" and "bar" won't show up as options
test-complete --does-not-complete <TAB>

# "foo" and "bar" show up as options
test-complete --completes <TAB>
bar foo

Workaround

Use HintOptions("foo", "bar") or HintAction(customFunc) when using an Enums type:

	nowCompletes = kingpin.Flag("does-not-complete", "this flag does not tab complete").HintOptions("foo", "bar").Enums("foo", "bar")

This works fine, but seems wholly unnecessary to repeat the values twice and means Enums still doesn't behave the way it is expected to.