add megaeth mainnet (#7714)

* add megaeth mainnet registration

* add mainnet designation

Author: yangl1996

_data/chains/eip155-4326.json

@@ -0,0 +1,16 @@
+{
+  "name": "MegaETH Mainnet",
+  "chain": "ETH",
+  "nativeCurrency": {
+    "name": "Ether",
+    "symbol": "ETH",
+    "decimals": 18
+  },
+  "rpc": [],
+  "faucets": [],
+  "explorers": [],
+  "infoURL": "https://megaeth.com",
+  "shortName": "megaeth",
+  "chainId": 4326,
+  "networkId": 4326
+}