chore: Prepare API for expression modernization

Author: pawamoy

src/griffe/expressions.py

@@ -108,6 +108,16 @@ def iterate(self, *, flat: bool = True) -> Iterator[str | Expr]:  # noqa: ARG002
         """
         yield from ()
 
+    def modernize(self) -> Expr:
+        """Modernize the expression.
+
+        For example, use PEP 604 type unions `|` instead of `typing.Union`.
+
+        Returns:
+            A modernized expression.
+        """
+        return self
+
     def as_dict(self, **kwargs: Any) -> dict[str, Any]:
         """Return the expression as a dictionary.