Add visibility parameters to control scope of generated extension

Sources/ToolCore/ExtensionVisibility.swift

@@ -0,0 +1,31 @@
+//
+//  ExtensionVisibility.swift
+//  ResgenSwift
+//
+//  Created by Thibaut Schmitt on 17/07/2025.
+//
+
+import ArgumentParser
+
+package enum ExtensionVisibility: String, CustomStringConvertible, ExpressibleByArgument {
+
+    case `public`
+    case `private`
+    case `internal`
+    case `package`
+
+    // MARK: - CustomStringConvertible
+
+    package var description: String {
+        switch self {
+        case .public:
+            "public"
+        case .private:
+            "private"
+        case .internal:
+            "internal"
+        case .package:
+            "package"
+        }
+    }
+}