Add tag component and test (#2761)

* Add tag component and test

* Remove 0 capacity

* Add tag component extensions

* Change tags to be prototypes

Co-authored-by: Vera Aguilera Puerto <6766154+Zumorica@users.noreply.github.com>

Content.Shared/Prototypes/Tag/TagPrototype.cs

@@ -0,0 +1,31 @@
+#nullable enable
+using JetBrains.Annotations;
+using Robust.Shared.Interfaces.Serialization;
+using Robust.Shared.Prototypes;
+using Robust.Shared.Serialization;
+using YamlDotNet.RepresentationModel;
+
+namespace Content.Shared.Prototypes.Tag
+{
+    /// <summary>
+    ///     Prototype representing a tag in YAML.
+    ///     Meant to only have an ID property, as that is the only thing that
+    ///     gets saved in TagComponent.
+    /// </summary>
+    [Prototype("Tag")]
+    public class TagPrototype : IPrototype, IIndexedPrototype, IExposeData
+    {
+        public string ID { get; [UsedImplicitly] private set; } = default!;
+
+        public void ExposeData(ObjectSerializer serializer)
+        {
+            serializer.DataField(this, x => x.ID, "id", "");
+        }
+
+        public void LoadFrom(YamlMappingNode mapping)
+        {
+            var serializer = YamlObjectSerializer.NewReader(mapping);
+            ExposeData(serializer);
+        }
+    }
+}