Fix Cloudflare Workers adapter target

[pzuraq]

When attempting to upload a SvelteKit app which contains modern ES
features such as optional chaining or nullish coalescing, workers is
unable to parse the built output.

Module parse failed: Unexpected token (13066:47)
You may need an appropriate loader to handle this file type, currently no loaders are configured to process this file. See https://webpack.js.org/concepts#loaders
|       if (typeof window !== "undefined") {
|         const windowAny = window;
>         const dataLayer = windowAny.dataLayer ?? [];
|         windowAny.dataLayer = dataLayer;
|         gtag = function() {
Error: webpack returned an error. You may be able to resolve this issue by running npm install

I could not find any official documentation of what version of ES
workers support, but ES2020 is the year that nullish coalescing was
introduced, and setting it to ES2019 appears to fix the issue.

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Unsure how to test this

Tests

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[changeset-bot]

🦋 Changeset detected

Latest commit: 4867f2d

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/adapter-cloudflare-workers	Patch

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[bluwy]

Why is webpack being used to bundle stuff in cloudflare workers? The docs recommend to set type = "javascript" in the wrangler config, so webpack shouldn't be used in the first place.

[pzuraq]

I was taking a look at it this morning and @bluwy appears to be correct, this app was converted from a previous Cloudflare Workers app that we changed to SvelteKit. We kept parts of the workers config, including the Webpack build portion, which is no longer necessary. Removing it also fixed the issue.

So I believe this could be reverted, sorry I was just typing/about to close!