macos: disable macOS-only API on iOS

macos/Sources/Helpers/String+Extension.swift

@@ -7,6 +7,7 @@ extension String {
         return self.prefix(maxLength) + trailing
     }
     
+    #if canImport(AppKit)
     func temporaryFile(_ filename: String = "temp") -> URL {
         let url = FileManager.default.temporaryDirectory
             .appendingPathComponent(filename)
@@ -15,4 +16,5 @@ extension String {
         try? string.write(to: url, atomically: true, encoding: .utf8)
         return url
     }
+    #endif
 }