revert: Make first, last orderable

Author: dangotbanned

narwhals/_plan/aggregation.py

@@ -29,14 +29,6 @@ def __repr__(self) -> str:
 class Count(Agg): ...
 
 
-class First(Agg):
-    """https://github.com/narwhals-dev/narwhals/issues/2526."""
-
-
-class Last(Agg):
-    """https://github.com/narwhals-dev/narwhals/issues/2526."""
-
-
 class Max(Agg): ...
 
 
@@ -79,6 +71,14 @@ class Var(Agg):
 class OrderableAgg(Agg): ...
 
 
+class First(OrderableAgg):
+    """https://github.com/narwhals-dev/narwhals/issues/2526."""
+
+
+class Last(OrderableAgg):
+    """https://github.com/narwhals-dev/narwhals/issues/2526."""
+
+
 class ArgMin(OrderableAgg): ...