adding isthmus timestamp (#1149)

Author: sbvegan

superchain/configs/mainnet/worldchain.toml

@@ -18,6 +18,7 @@ max_sequencer_drift = 1800
   fjord_time = 1721826000 # Wed 24 Jul 2024 13:00:00 UTC
   granite_time = 1727780400 # Tue 1 Oct 2024 11:00:00 UTC
   holocene_time = 1738238400 # Thu 30 Jan 2025 12:00:00 UTC
+  isthmus_time = 1764072000 # Tue 25 Nov 2025 12:00:00 UTC
 
 [optimism]
   eip1559_elasticity = 10