enforces CDK-specific linter rules

All linter rules live in rules.ts. To create a new rule, create a new class in that file that extends ValidationRule. That class will be automatically picked up by pkglint when it is run.

When designing a new rule, do not assume that pkglint will run on a built aws-cdk repository. pkglint is sometimes run on just the source code of aws-cdk modules so validating compiled code will result in failure.