Add scheduler, fix imports and exports

[ci skip]

nyxx.extensions/lib/scheduled_event.dart

@@ -0,0 +1 @@
+export "src/scheduler/scheduler.dart";

nyxx.extensions/lib/src/scheduler/scheduler.dart

@@ -0,0 +1,27 @@
+import "dart:async";
+
+import "package:nyxx/nyxx.dart" show Nyxx;
+
+/// Callback for [ScheduledEvent]. Is executed every period given in [ScheduledEvent]
+typedef ScheduledEventCallback = Future<void> Function(Nyxx, Timer);
+
+/// Creates and starts new periodic event. [callback] will be executed every given duration of time.
+/// Event can be stopped via [stop] function.
+class ScheduledEvent {
+  /// [Nyxx] instance
+  final Nyxx client;
+
+  /// Callback which will be run every given period of time
+  final ScheduledEventCallback callback;
+  
+  late final Timer _timer;
+
+  /// Creates and starts new periodic event. [callback] will be executed every [duration] of time.
+  /// Event can be stopped via [stop] function.
+  ScheduledEvent(this.client, Duration duration, this.callback) {
+    _timer = Timer.periodic(duration, (timer) => callback(client, timer));
+  }
+
+  /// Stops [ScheduledEvent] if running.
+  void stop() => _timer.cancel();
+}

nyxx.extensions/lib/utils.dart

@@ -1,3 +1 @@
-library nyxx.extensions.utils;
-
 export "src/utils.dart";