chore(cfnspec): Add coverage directory to npmignore (backport #20016) (#20931)

This is an automatic backport of pull request #20016 done by [Mergify](https://mergify.com).


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Author: mergify[bot]

packages/@aws-cdk/cfnspec/.npmignore

@@ -12,3 +12,4 @@ test/
 jest.config.js
 **/*.integ.snapshot
 **/*.integ.snapshot
+coverage/