Extend the optional delegate to accept a list of hardcoded acceptable values

[NoakPalander]

I propose to add the following overloads to the ArgParser@option function, so that we can specify a list of valid values, and if the user specifies anything else, it'll throw. Example usage would be

val mode by parser.option<Mode?>(
        "-m", "--mode",
        values = listOf("play", "search", "list"),
        help = "some very nice description, valid options are: [id, name, flags]",
    ) {
        when(this) {
            "play" -> Mode.Play
            "name" -> Mode.Search
            "tags" -> Mode.List
            else -> null
        }
    }.default(null)

or with the latter overload

val mode by parser.option(
        "-m", "--mode",
        values = listOf("play", "search", "list"),
        help = "some very nice description, valid options are: [play, search, list]",
    ).default<String?>(null)

This would allow usage to be as follows:

$ ./app --mode=play          // ok
$ ./app -m play              // ok
$ ./app -m hello             // error, "hello" was not a valid value
$ ./app                      // ok, the option has a default value and isnt a required field

It's technically already possible through the already existing overloads, but I can definitely see this to be a very common use, so I find it valuable enough to be included in the library.