always generate 404.html (#9294)

* add fallback option to adapter-cloudflare

* always generate fallback

* Update packages/adapter-cloudflare/index.js

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

* format

---------

Co-authored-by: ajgeiss0702 <ajgeiss72@gmail.com>
Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

.changeset/long-lobsters-love.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-cloudflare': minor
+---
+
+feat: generate `404.html`

packages/adapter-cloudflare/index.js

@@ -1,5 +1,5 @@
 import { writeFileSync } from 'node:fs';
-import { posix } from 'node:path';
+import * as path from 'node:path';
 import { fileURLToPath } from 'node:url';
 import * as esbuild from 'esbuild';
 
@@ -16,11 +16,14 @@ export default function (options = {}) {
 			builder.rimraf(tmp);
 			builder.mkdirp(tmp);
 
+			// generate 404.html first which can then be overridden by prerendering, if the user defined such a page
+			await builder.generateFallback(path.join(dest, '404.html'));
+
 			const dest_dir = `${dest}${builder.config.kit.paths.base}`;
 			const written_files = builder.writeClient(dest_dir);
 			builder.writePrerendered(dest_dir);
 
-			const relativePath = posix.relative(tmp, builder.getServerDirectory());
+			const relativePath = path.posix.relative(tmp, builder.getServerDirectory());
 
 			writeFileSync(
 				`${tmp}/manifest.js`,