Zombie Mode [New Game Mode] (#8501)

Co-authored-by: Kara <lunarautomaton6@gmail.com>
Co-authored-by: metalgearsloth <comedian_vs_clown@hotmail.com>

Content.Shared/Zombies/ZombieEvents.cs

@@ -0,0 +1,25 @@
+using Content.Shared.Actions;
+
+namespace Content.Shared.Zombies;
+
+/// <summary>
+///     Event that is broadcast whenever an entity is zombified.
+///     Used by the zombie gamemode to track total infections.
+/// </summary>
+public readonly struct EntityZombifiedEvent
+{
+    /// <summary>
+    ///     The entity that was zombified.
+    /// </summary>
+    public readonly EntityUid Target;
+
+    public EntityZombifiedEvent(EntityUid target)
+    {
+        Target = target;
+    }
+};
+
+/// <summary>
+///     Event raised when a player zombifies themself using the "turn" action
+/// </summary>
+public sealed class ZombifySelfActionEvent : InstantActionEvent { };