Adds generic damage visualizer to the game (#4893)

Content.Client/Damage/DamageVisualizerComponent.cs

@@ -0,0 +1,26 @@
+using System.Collections.Generic;
+using Robust.Shared.GameObjects;
+
+namespace Content.Client.Damage
+{
+    // Stores all the data for a DamageVisualizer.
+    //
+    // Storing it inside of the AppearanceComponent's data
+    // dictionary was too messy, but at least we can
+    // store it in the entity itself as a separate,
+    // dynamically added component.
+    [RegisterComponent]
+    public class DamageVisualizerDataComponent : Component
+    {
+        public override string Name => "DamageVisualizerData";
+
+        public List<object> TargetLayerMapKeys = new();
+        public bool Disabled = false;
+        public bool Valid = true;
+        public int LastDamageThreshold = 0;
+        public Dictionary<object, bool> DisabledLayers = new();
+        public Dictionary<object, string> LayerMapKeyStates = new();
+        public Dictionary<string, int> LastThresholdPerGroup = new();
+        public string TopMostLayerKey = default!;
+    }
+}