add more files and folders to `.npmignore`

## Reproduction steps
Download tarball from this url:
```shell
$ npm view graphql-subscriptions dist.tarball
```
List content of the file
```shell
$ tar -tf graphql-subscriptions-3.0.0.tgz
package/AUTHORS
package/CHANGELOG.md
package/.circleci/config.yml
package/CODEOWNERS
package/CONTRIBUTING.md
package/.designs/authorization.md
package/dist/index.d.ts
package/dist/index.js
package/dist/index.js.map
package/dist/pubsub-async-iterable-iterator.d.ts
package/dist/pubsub-async-iterable-iterator.js
package/dist/pubsub-async-iterable-iterator.js.map
package/dist/pubsub.d.ts
package/dist/pubsub-engine.d.ts
package/dist/pubsub-engine.js
package/dist/pubsub-engine.js.map
package/dist/pubsub.js
package/dist/pubsub.js.map
package/dist/with-filter.d.ts
package/dist/with-filter.js
package/dist/with-filter.js.map
package/.github/ISSUE_TEMPLATE.md
package/.github/PULL_REQUEST_TEMPLATE.md
package/LICENSE
package/package.json
package/README.md
package/.travis.yml
```

Expected result
- Folders and files not needed for the package to work should be ignored when publishing the package.

Actual result
- Unneeded files are included in the tarball like `.github/`, `.circleci/`, `.designs/`, `.travis.yml` and more

Update the repository’s .npmignore based on the unnecessary files and folders shown in the issue, including .github/, .circleci/, .designs/, and .travis.yml. Run npm pack or inspect the generated tarball with tar -tf and confirm that package runtime files remain while repository-only files are excluded.

Written by the indexing model from the issue text.