feat: add playback seeking

tests/test_music_player.py

@@ -103,6 +103,16 @@ class MusicPlayerQueueTest(unittest.IsolatedAsyncioTestCase):
         self.assertEqual(state.current.id, "b")
         self.assertEqual(state.queue[0].plays_left, 4)
 
+    async def test_seek_restarts_current_track_at_requested_position(self):
+        player, voice_client, state = await self._queue_abc()
+
+        position = await player.seek(GUILD, 4.5)
+
+        self.assertEqual(position, 4.5)
+        self.assertEqual(state.current.id, "a")
+        self.assertEqual(state.current.elapsed, 4.5)
+        self.assertIs(voice_client.playing, state.current)
+
     async def test_loop_track_replays_same_index(self):
         player, _, state = await self._queue_abc()
         await player.play_next(GUILD, advance=True)