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

Installing cfnspec shows the coverage directory is being published to npm. I assume this should be excluded since build artifacts aren't typically published, and at a glance most CDK packages exclude any build artifacts from the published package. (If there is a specific reason why the coverage folder is being published, please advise since I'd be interested to understand the use case.)

```
$ npm install @aws-cdk/cfnspec
$ ls -1 node_modules/@aws-cdk/cfnspec/
CHANGELOG.md
LICENSE
coverage
lib
package.json
...
$  ls -1 node_modules/@aws-cdk/cfnspec/coverage
base.css
block-navigation.js
build-tools
favicon.png
index.html
junit.xml
...
```
----

### All Submissions:

* [x] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/master/CONTRIBUTING.md)

### Adding new Unconventional Dependencies:

* N/A

### New Features

* N/A

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/cfnspec/.npmignore

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