feat(Grid): flexible at() method arguments

- Support tuple argument: grid.at((x, y))
- Support keyword arguments: grid.at(x=5, y=3)
- Support pos keyword: grid.at(pos=(2, 8))
- Maintain backward compatibility with grid.at(x, y)
- Add comprehensive error handling for invalid arguments

Improves API ergonomics and Python-like flexibility

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

_test.py

@@ -0,0 +1,16 @@
+import mcrfpy
+
+# Create a new scene
+mcrfpy.createScene("intro")
+
+# Add a text caption
+caption = mcrfpy.Caption((50, 50), "Welcome to McRogueFace!")
+caption.size = 48 
+caption.fill_color = (255, 255, 255)
+
+# Add to scene
+mcrfpy.sceneUI("intro").append(caption)
+
+# Switch to the scene
+mcrfpy.setScene("intro")
+