Add health overlay and a command to toggle it (#3278)

* Add health overlay bar and a command to toggle it

* Remove empty line

Content.Client/Commands/ToggleHealthOverlayCommand.cs

@@ -0,0 +1,21 @@
+using Content.Client.GameObjects.EntitySystems.HealthOverlay;
+using Robust.Shared.Console;
+using Robust.Shared.GameObjects;
+
+namespace Content.Client.Commands
+{
+    public class ToggleHealthOverlayCommand : IConsoleCommand
+    {
+        public string Command => "togglehealthoverlay";
+        public string Description => "Toggles a health bar above mobs.";
+        public string Help => $"Usage: {Command}";
+
+        public void Execute(IConsoleShell shell, string argStr, string[] args)
+        {
+            var system = EntitySystem.Get<HealthOverlaySystem>();
+            system.Enabled = !system.Enabled;
+
+            shell.WriteLine($"Health overlay system {(system.Enabled ? "enabled" : "disabled")}.");
+        }
+    }
+}