Playground block: Add ARIA labels to icon-only buttons

[brandonpayton]

What?

This PR adds ARIA labels to the code editor's "Add File" and "Download Code as a Zip File" buttons which previously only included visual indications of their function.

Fixes #286

Why?

Without ARIA labels, people using assistive technology may have no way to know what the buttons do.

Testing Instructions

• npx nx run wordpress-playground-block:dev
• Create a new post
• Add a Playground block in the editor
• Use a screen reader to engage with the "Add File" and "Download Code as Zip File" button and confirm the labels are recognized
• Publish the post and use a screen reader to confirm the "Download Code as Zip File" button has a recognized label

[brandonpayton]

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