Support File / Path mappings

[AndreasVolkmann]

Would be nice to be able to map to File or Path out of the box.
Also makes it easier to validate arguments.

For example, I have a configuration that takes a string path to a directory:

data class Configuration(
    val kindlePath: String,
) {
    init {
        require(kindlePath.isNotBlank()) { "Provided Kindle path cannot be blank" }
    }
}

Later on, I convert it to a Path.