Add the concept of "available peripherals" to IComputerAccess

This provides a mechanism for peripherals to see what else a computer is
connected to - and then interact with those peripherals.

We also add the ability to query what block or tile a peripheral
targets. This allows one to interact with the original block of adjacent
peripherals instead.

src/main/java/dan200/computercraft/api/peripheral/IPeripheral.java

@@ -114,6 +114,18 @@ public interface IPeripheral
     {
     }
 
+    /**
+     * Get the object that this peripheral provides methods for. This will generally be the tile entity
+     * or block, but may be an inventory, entity, etc...
+     *
+     * @return The object this peripheral targets
+     */
+    @Nonnull
+    default Object getTarget()
+    {
+        return this;
+    }
+
     /**
      * Determine whether this peripheral is equivalent to another one.
      *