feat: Implement no-op extension

Author: pawamoy

src/griffe_runtime_objects/__init__.py

@@ -5,4 +5,6 @@
 
 from __future__ import annotations
 
-__all__: list[str] = []
+from griffe_runtime_objects._internals.extension import RuntimeObjectsExtension
+
+__all__: list[str] = ["RuntimeObjectsExtension"]

src/griffe_runtime_objects/_internals/__init__.py

@@ -0,0 +1,5 @@
+import griffe
+
+
+class RuntimeObjectsExtension(griffe.Extension):
+    """Store runtime objects in Griffe objects' `extra` attribute."""

src/griffe_runtime_objects/_internals/extension.py

@@ -0,0 +1,6 @@
+"""Test extension."""
+
+
+def test_extension() -> None:
+    """Dummy test."""
+    assert True