Swift 6.4 migration and exhancements (#1)

This PR contains the work done to update the library and the attached example project to use the Swift 6.4 computer as a minimum supported version and also, to use the latest features introduced in it.

Reviewed-on: #1
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>

Examples/HelloPlaydate/Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version: 6.3
+// swift-tools-version: 6.4
 
 import PackageDescription
 

Examples/HelloPlaydate/Sources/HelloPlaydate/Game.swift

@@ -8,7 +8,7 @@
 import CPlaydate
 import PlaydateKit
 
-@_cdecl("eventHandler")
+@c(eventHandler)
 public func eventHandler(
     pointer: UnsafeMutableRawPointer,
     event: PDSystemEvent,