Freeze loglevel enum (#255)

* Use @Frozen for loglevel enum

* Add public accessibility for loggingcallback and loglevel

Author: ZachNagengast

Sources/WhisperKit/Core/Utils.swift

@@ -762,15 +762,16 @@ public func getMemoryUsage() -> UInt64 {
 
 // MARK: - Singletons
 
-public class Logging {
-    static let shared = Logging()
-    var logLevel: LogLevel = .none
+open class Logging {
+    public static let shared = Logging()
+    public var logLevel: LogLevel = .none
 
     public typealias LoggingCallback = (_ message: String) -> Void
-    var loggingCallback: LoggingCallback?
+    public var loggingCallback: LoggingCallback?
 
     private let logger = OSLog(subsystem: Bundle.main.bundleIdentifier ?? "com.argmax.whisperkit", category: "WhisperKit")
 
+    @frozen
     public enum LogLevel: Int {
         case debug = 1
         case info = 2