chore: add mistakenly dropped cfn-parse.ts to core (aws#19804)

This should have been a part of aws#19773. Ensures that old imports of `@aws-cdk/core/lib/cfn-parse` still work, so that it is not a breaking change. 

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packages/@aws-cdk/core/lib/cfn-parse.ts

@@ -0,0 +1 @@
+export * from './helpers-internal';