fix: pass useNatInstance prop to MainStack (#71)

## Problem

The `useNatInstance` property was declared in `EnvironmentProps` and set
in the `props` object, but it was not being passed to the `MainStack`
constructor.

## Solution

Added `useNatInstance: props.useNatInstance` to the `MainStack`
constructor call.

## Changes

- Modified `cdk/bin/cdk.ts` to pass the `useNatInstance` prop to
`MainStack`

This ensures that the `useNatInstance` configuration is properly
propagated to the main stack, allowing users to control whether to use
NAT instances or NAT gateways.

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Co-authored-by: remote-swe-app[bot] <123456+remote-swe-app[bot]@users.noreply.github.com>

Author: konokenj

cdk/bin/cdk.ts

@@ -46,6 +46,7 @@ new MainStack(app, 'ServerlessWebappStarterKitStack', {
   crossRegionReferences: true,
   sharedCertificate: virginia.certificate,
   domainName: props.domainName,
+  useNatInstance: props.useNatInstance,
   signPayloadHandler: virginia.signPayloadHandler,
 });