fix: replace bigint exponentiation to fix Turbopack compatibility

Replace `10n ** 18n` with `BigInt(1e18)` in estimateErc20PaymasterCost.

Turbopack does not yet support the `**` operator with bigint operands
("right-associative binary expression not yet implemented"). This is
the same pattern already used on the adjacent line 93.

Closes #499

Author: skyc1e

packages/permissionless/actions/pimlico/estimateErc20PaymasterCost.ts

@@ -93,7 +93,7 @@ export const estimateErc20PaymasterCost = async <
     const costInToken = (maxCostInWei * exchangeRate) / BigInt(1e18)
 
     // represents the userOperation's max cost in usd (with 6 decimals of precision)
-    const costInUsd = (maxCostInWei * exchangeRateNativeToUsd) / 10n ** 18n
+    const costInUsd = (maxCostInWei * exchangeRateNativeToUsd) / BigInt(1e18)
 
     return {
         costInToken,