Playground block: Stop monopolizing main element

[brandonpayton]

What?

Fixes #292 - "Playground block: A11Y: Remove duplicate main landmarks"

This PR replaces the Playground block's <main> element with a <section> element annotated with an aria-label.

Why?

There is only supposed to be one <main> element per document. We do not know if a document including a Playground block will be rendered within another <main> element, and there can be multiple Playground blocks in a single document.

Testing Instructions

• npx nx run wordpress-playground-block:dev
• Create a new post
• Add a Playground block in the editor and confirm it renders normally
• Publish the post and confirm the Playground block renders normally on the front end
• Use screen reader and confirm that the section label is noted when engaging with the section

[brandonpayton]

I'm going to merge this because it has been tested and is low risk.