Portals & hand teleporter (#13266)

* basic system with portals & linked ents

* hand tele sprites, no impl

* hand tele and teleportation works

* fancy it up

* oog

* special case projectiles

* predict portal-to-portal teleportation

* this stuff

* check nullspace

* sloth

* give to rd instead

* i guess this can probably happen

* docs

Content.Shared/Teleportation/Components/LinkedEntityComponent.cs

@@ -0,0 +1,42 @@
+using Content.Shared.Teleportation.Systems;
+using Robust.Shared.GameStates;
+using Robust.Shared.Serialization;
+
+namespace Content.Shared.Teleportation.Components;
+
+/// <summary>
+///     Represents an entity which is linked to other entities (perhaps portals), and which can be walked through/
+///     thrown into to teleport an entity.
+/// </summary>
+[RegisterComponent, Access(typeof(LinkedEntitySystem)), NetworkedComponent]
+public sealed class LinkedEntityComponent : Component
+{
+    /// <summary>
+    ///     The entities that this entity is linked to.
+    /// </summary>
+    [DataField("linkedEntities")]
+    public HashSet<EntityUid> LinkedEntities = new();
+
+    /// <summary>
+    ///     Should this entity be deleted if all of its links are removed?
+    /// </summary>
+    [DataField("deleteOnEmptyLinks")]
+    public bool DeleteOnEmptyLinks = false;
+}
+
+[Serializable, NetSerializable]
+public sealed class LinkedEntityComponentState : ComponentState
+{
+    public HashSet<EntityUid> LinkedEntities;
+
+    public LinkedEntityComponentState(HashSet<EntityUid> linkedEntities)
+    {
+        LinkedEntities = linkedEntities;
+    }
+}
+
+[Serializable, NetSerializable]
+public enum LinkedEntityVisuals : byte
+{
+    HasAnyLinks
+}