Implement EntityCollection.extend() method for Issue #27

Added extend() method to EntityCollection that accepts any iterable
of Entity objects and adds them all to the collection. The method:
- Accepts lists, tuples, generators, or any iterable
- Validates all items are Entity objects
- Sets the grid association for each added entity
- Properly handles errors and empty iterables

closes #27

quick_extend_test.py

@@ -0,0 +1,20 @@
+import mcrfpy
+
+# Create grid
+t = mcrfpy.Texture("assets/kenney_ice.png", 16, 16)
+g = mcrfpy.Grid(5, 5, t, (0, 0), (100, 100))
+
+# Create some entities
+entities = [mcrfpy.Entity((i, i), t, i, g) for i in range(3)]
+
+# Test extend
+print(f"Initial entities: {len(g.entities)}")
+g.entities.extend(entities)
+print(f"After extend: {len(g.entities)}")
+
+# Test with tuple
+more = (mcrfpy.Entity((3, 3), t, 3, g), mcrfpy.Entity((4, 4), t, 4, g))
+g.entities.extend(more)
+print(f"After second extend: {len(g.entities)}")
+
+print("✓ EntityCollection.extend() works!")