Upgrades to the latest version of Acorn, and replaces ecmaVersion: 16 with 'latest' which allows Svelte code to use the using keyword (which is now stage 4, i.e. part of the language).

I didn't think it worthwhile to add a test (would be like having a test for var declarations or whatever) but you can easily see it working in the playground once it deploys.

One thing we could conceivably do is transform components with top-level using such that disposals don't occur until the component is removed. This would mean wrapping everything in an async function that ends with something like this:

return new Promise((fulfil) => {
  $effect(() => fulfil);
});

Personally I'm not sold on the idea — it's a little magical, and would prevent the component from exporting values — but I raise it as a possibility for discussion. If we thought we might want to add that capability in future we could always prevent the usage of top-level using, though that itself might be annoying.

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