fix: Adds multis to map iterators, fixes searching empty nested containers (#1581)

### Summary
Eliminates `IPooledEnumerable<BaseMulti>` and `eable.Free()` from `Map` for multis. This drastically simplifies code that iterates in range, for example:

```cs
foreach (var m in m.GetMultisInRange(5))
{
}
```
The code above no longer requires an eable and calling `Free()`.


### Bug Fixes
- [X] Fixes an issue with searching through nested empty containers.

Projects/UOContent/Multis/Boats/BaseBoat.cs

@@ -243,11 +243,9 @@ namespace Server.Multis
 
         public static BaseBoat FindBoatAt(Point3D loc, Map map)
         {
-            var sector = map.GetSector(loc);
-
-            for (var i = 0; i < sector.Multis.Count; i++)
+            foreach (var boat in map.GetMultisAt<BaseBoat>(loc))
             {
-                if (sector.Multis[i] is BaseBoat boat && boat.Contains(loc.X, loc.Y))
+                if (boat.Contains(loc.X, loc.Y))
                 {
                     return boat;
                 }