docs: use new Svelte 5 event handler syntax on preview site examples

[F-fengzi]

Not sure how long this fix to the preview site will be relevant now that Svelte 5 is out (congrats! 🎉) and we are moving to the new omnisite, but some REPL examples of the new syntaxes in the "Old vs new" page still has

<button on:click={() => count++}>...</button>

in them despite diffed examples embedded in the doc itself already properly reflected the changes. The links are now updated to show

<button onclick={() => count++}>...</button>

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[dummdidumm]

Thank you!