fix clearStorage function to match expected behaviour

[seralichtenhahn]

Resolves #7003

Changes: The clearStorage() function previously cleared the entire localStorage including the data set outside of p5.js. This is not the expected behaviour, the documentation states that only data set by the storeItem() will get cleared. This fix checks which data has been set with storeItem() and only removes the relevant data.

• updated clearStorage() function
• added unit test

PR Checklist

• [x] npm run lint passes
• [x] [Inline documentation] is included / updated
• [x] [Unit tests] are included / updated

[welcome[bot]]

🎉 Thanks for opening this pull request! Please check out our contributing guidelines if you haven't already. And be sure to add yourself to the list of contributors on the readme page!

[limzykenneth]

Looks good. Thanks!