fix(synthetics): include auto-delete-underlying-resources in package (#…

…27096)

The source code in `custom-resource-handlers` was not included in the `npm pack` and thus unavailable to customers. This PR includes it. Tested by running `npm pack` and confirming its existence and also testing it the resulting package on a local cdk app.

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packages/@aws-cdk/aws-synthetics-alpha/.npmignore

@@ -21,9 +21,13 @@ tsconfig.json
 
 .eslintrc.js
 jest.config.js
+
 # exclude cdk artifacts
 **/cdk.out
 junit.xml
 test/
 !*.lit.ts
 **/*.snapshot
+
+# include custom-resource-handlers
+!custom-resource-handlers/*