Add onboarding modal to playground

[n8finch]

Resolves #497

Motivation for the change, related issues

Adds an onboarding modal using the Modal from WordPress Components.

Testing Instructions (or ideally a Blueprint)

1. Checkout the branch
2. Run npm run dev
3. Make sure you've cleared any hideExperimentalNotice cookie from your Application cache
4. When it initially loads, you should see a modal pop up like the below screenshot.
5. When you click on the blue "ok" button, it should hide and create a cookie to hide the modal.

Feedback

@adamziel I worked on this with @sejas , but wanted to check with you specifically on two pieces:

1. Do we want the cookie to be implemented "forever", or to expire in a week or month? Current implementation is forever.
2. Do you have any feedback on the copy? This can definitely say whatever we like, but I'm happy to change it to be whatever you think is appropriate, as well as including or removing emojis 😁.

[n8finch]

Ahh, seeing that the Cypress was failing tests, I updated the website-ui tests and added a URL param check. Looks like things are good now. 🙂

[adamziel]

👋 Thank you for contributing @n8finch! It pains me to say this – the onboarding modal idea will soon be obsoleted by this work: #1731. It will enable saving Playgrounds, solve the data loss problem, and introduce a sidebar where any required onboarding could happen. We should have closed the original issue earlier, sorry about that!

[n8finch]

@adamziel haha, no worries at all, it was a good learning experience, good to work with @sejas at the contributor day, and I'm not emotionally attached to code I so it's all good 😁.

Keep up the good work!