Playground / REPL enhancements

Describe the problem

Based on a Twitter/X discussion, and at the request of @Rich-Harris, I would like to start a discussion on possibly adding some additional features/functionality to the playaround (aka REPL). I thought opening a general issue to discuss features/scope, and then spiking off individual issues as we determine their usefulness and feasibility.

Initial ideas:

• Full page / presentation mode (with URL)
  • Remove top header
  • Show/hide code, preview, console, etc
• Screenshot / previews (similar to codepen, codesandbox, observable notebooks (🫗), etc)
• Ability to browse all of a user's REPLs, and/or the ability to "publish" / mark as public
  • Currently the security model is similar to "Unlisted" on YouTube where anyone can see a REPL if you have the link, and doesn't warrant itself to exploration
  • Going further with "new", "trending", etc would be nice, but likely well outside the initial scope.

Describe the proposed solution

Discuss these (and other ideas) to determine their usefulness, feasibility, and possible design/architecture.

Importance

nice to have

Start by reviewing the linked discussion and request, then examine the existing playground/REPL experience. Define the useful and feasible scope for presentation mode, screenshots or previews, and REPL discovery before splitting work into focused issues; done is a decided scope and architecture rather than an implementation from this issue alone.