Placeholder API expansion wrapper

[Matt-MX]

Implementation of a wrapper for placeholder requests.

placeholderExpansion {
    name = "ktgui"
    version = "1.0.0"

    // Reuse arguments from declarative commands
    val player by onlinePlayerArgument()
    matches("uuid" / player) {
        player().uniqueId.toString()
    }
    matches("uuid") {
        requesting.uniqueId.toString()
    }
}

This allows for a more declarative and streamlined approach to supplying placeholders to PlaceholderAPI