Fiddle: dropdown for Mitosis versions

[samijaber]

Instead of hardcoding mitosis versions in Fiddle, we should have a dropdown that dynamically fetches all (or the last 10-15) versions of the core package, and allow the user to select which version they want.

That means we won't need to publish the Fiddle anymore for new Core deploys.

Should probably do this after #478 is done.