feat: support Cloudflare Workers Builds

[eltigerchino]

Inspired by unjs/std-env#156

This PR detects the WORKERS_CI environment variable so that adapter-auto and adapter-cloudflare can work with Cloudflare's Workers Git integration to build the app without requiring a wrangler configuration from the user.

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@sveltejs/adapter-cloudflare	Minor
@sveltejs/adapter-auto	Minor

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[eltigerchino]

@benmccann I'm not sure if the adapter-auto case for Workers Builds is very useful with this PR because it doesn't "just work" like the other platforms do (since it fails without a Wrangler config file). Would it be useful to generate a basic Wrangler config during the Workers CI so that it just works? Or is it better that users get a deployment error which forces them to setup a wrangler.jsonc file?

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-13733-svelte.vercel.app/

[benmccann]

Ah, I guess the PR description is outdated then as it suggests it works without a config

What would a basic config contain? Is there anything we might accidentally omit or get wrong by doing that?

[eltigerchino]

Ah, I guess the PR description is outdated then as it suggests it works without a config

Yeah, I was half-way implementing the generated config but then wasn't sure if it would be a good idea.

What would a basic config contain? Is there anything we might accidentally omit or get wrong by doing that?

The minimum we would need would be:

{
	// `name` is not required since Cloudflare's dashboard requires the user to enter that before the build
	"main": ".svelte-kit/cloudflare/_worker.js",
	// this will be based on the current date
	"compatibility_date": "2025-04-24",
	"assets": {
		"binding": "ASSETS",
		"directory": ".svelte-kit/cloudflare",
	},
	// only required if the user uses `getRequestEvent()` asynchronously.
	// Enabling this doesn't add any polyfills like `nodejs_compat` does so it could be safe to always have this?
	// Otherwise we could only include it if we detect `getRequestEvent()` being used.
	"compatibility_flags": ["nodejs_als"]
}

Another question is: how should we generate the Wrangler config? There are currently two ways to do this:

1. Simulate a user's Wrangler configuration file.
   This is probably the easiest and means writing a new wrangler.jsonc if there is no user configuration file already.

2. Create a redirected Wrangler configuration.
   I'm a bit wary of using this method because if the redirected config exists, the Wrangler deploy and dev commands will always ignore the user's configuration file until the redirect is deleted. Fortunately, Wrangler does warn the user if the redirect config is being used. However, it would be better if we could create a config that acts as a fallback/override rather than a complete replacement. I've asked the Cloudflare team about this option and they recommended validating the user config instead.