Add PDA Ringtones (#5842)

Co-authored-by: metalgearsloth <comedian_vs_clown@hotmail.com>

Content.Shared/PDA/SharedRingerSystem.cs

@@ -0,0 +1,29 @@
+using System;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Serialization;
+
+namespace Content.Shared.PDA;
+
+public abstract class SharedRingerSystem : EntitySystem
+{
+    public const int RingtoneLength = 4;
+    public const int NoteTempo = 300;
+    public const float NoteDelay = 60f / NoteTempo;
+}
+
+[Serializable, NetSerializable]
+public enum Note : byte
+{
+    A,
+    Asharp,
+    B,
+    C,
+    Csharp,
+    D,
+    Dsharp,
+    E,
+    F,
+    Fsharp,
+    G,
+    Gsharp
+}