fix(houses): Converts house packets (1 of 2) (#502)

- [X] Adds `ToSpan()` for SpanWriter.
- [X] Moves house files to UOContent/Multis/Houses
- [X] Adds house packets

### SpanWriter.ToSpan()
`ToSpan()` returns an `SpanOwner` struct which allows access to the buffer as a Span by calling `Span`.

Example:
```cs
public SpanOwner CreatePacket()
{
  var writer = new SpanWriter(0x400);
  // write some stuff
  return writer.ToSpan();
}
```
In this example, we are creating a packet and returning the `SpanOwner` for caching. This means in the future we can do something like this:
```cs
ns.Send(_cachedPacket.Span);
```
Where `_cachedPacket` is the `SpanOwner`

Notes:
Do not use a SpanWriter, or attempt to dispose of it after calling `ToSpan()`. Doing so will probably cause an NPE.

Projects/UOContent/Multis/Houses/HousePackets.cs

@@ -0,0 +1,84 @@
+/*************************************************************************
+ * ModernUO                                                              *
+ * Copyright (C) 2019-2021 - ModernUO Development Team                   *
+ * Email: hi@modernuo.com                                                *
+ * File: HousePackets.cs                                                 *
+ *                                                                       *
+ * This program is free software: you can redistribute it and/or modify  *
+ * it under the terms of the GNU General Public License as published by  *
+ * the Free Software Foundation, either version 3 of the License, or     *
+ * (at your option) any later version.                                   *
+ *                                                                       *
+ * You should have received a copy of the GNU General Public License     *
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>. *
+ *************************************************************************/
+
+using System.Buffers;
+using Server.Network;
+
+namespace Server.Multis
+{
+    public static class HousePackets
+    {
+        private const int MaxItemsPerStairBuffer = 750;
+
+        public static void SendBeginHouseCustomization(this NetState ns, Serial house)
+        {
+            if (ns == null)
+            {
+                return;
+            }
+
+            var writer = new SpanWriter(stackalloc byte[17]);
+            writer.Write((byte)0xBF); // Packet Id
+            writer.Write((ushort)17);
+            writer.Write((short)0x20); // Sub-packet
+            writer.Write(house);
+            writer.Write((byte)0x04); // command
+            writer.Write((ushort)0x0000);
+            writer.Write((ushort)0xFFFF);
+            writer.Write((ushort)0xFFFF);
+            writer.Write((byte)0xFF);
+
+            ns.Send(writer.Span);
+        }
+
+        public static void SendEndHouseCustomization(this NetState ns, Serial house)
+        {
+            if (ns == null)
+            {
+                return;
+            }
+
+            var writer = new SpanWriter(stackalloc byte[17]);
+            writer.Write((byte)0xBF); // Packet Id
+            writer.Write((ushort)17);
+            writer.Write((short)0x20); // Sub-packet
+            writer.Write(house);
+            writer.Write((byte)0x05); // command
+            writer.Write((ushort)0x0000);
+            writer.Write((ushort)0xFFFF);
+            writer.Write((ushort)0xFFFF);
+            writer.Write((byte)0xFF);
+
+            ns.Send(writer.Span);
+        }
+
+        public static void SendDesignStateGeneral(this NetState ns, Serial house, int revision)
+        {
+            if (ns == null)
+            {
+                return;
+            }
+
+            var writer = new SpanWriter(stackalloc byte[13]);
+            writer.Write((byte)0xBF); // Packet Id
+            writer.Write((ushort)13);
+            writer.Write((short)0x1D); // Sub-packet
+            writer.Write(house);
+            writer.Write(revision);
+
+            ns.Send(writer.Span);
+        }
+    }
+}