feat: add reset button to REPL

[7nik]

Closes #1126
The button just re-mounts the App, but maybe re-instantiating the entire iframe, to clean up forgotten timers etc., would be better.
The button location isn't the best one - there is no address bar, like in the tutorial or SvelteLab, so alternatives are the console head and before the buttons Fork and Save the REPL. But I find them a bit too far from the tab.

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Name	Status	Preview	Updated (UTC)
svelte-dev	✅ Ready (Inspect)	Visit Preview	Apr 30, 2025 11:20pm