fix(app): add source map support to TypeScript apps

Add source map support to the TypeScript app entry point template.
This won't retroactively fix existing CDK apps, but it will ensure
that apps generated in the future via `cdk init --app` will have
it enabled.

Fixes #1579

packages/aws-cdk/lib/init-templates/app/typescript/bin/%name%.template.ts

@@ -1,4 +1,5 @@
 #!/usr/bin/env node
+import 'source-map-support/register';
 import cdk = require('@aws-cdk/cdk');
 import { %name.PascalCased%Stack } from '../lib/%name%-stack';