Use wasm.wordpress.net as staging

[adamziel]

Playground has no staging environment at the moment which makes testing more difficult.

However, the Playground demo is available under two different domains:

• https://playground.wordpress.net/
• https://wasm.wordpress.net/

Let's reconfigure that server to serve a separate, staging site on wasm.wordpress.net

Let's create separate issues to track each item from the checklist below.

Done is

• https://wasm.wordpress.net/ serves files from a separate staging directory on the server
• Deployment-related GitHub actions release to that staging directory
• There's a new GitHub action to promote staging to production