feat: generate .assetsignore file for Cloudflare deployment

.changeset/proud-taxis-admire.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-cloudflare': minor
+---
+
+feat: add `.assetsignore` file to avoid serving known server-only files to browser

packages/adapter-cloudflare/.gitignore

@@ -1 +1,2 @@
-/files
+/files/*
+!/files/.assetsignore

packages/adapter-cloudflare/files/.assetsignore

@@ -0,0 +1,4 @@
+_worker.js
+_routes.json
+_headers
+_redirects

packages/adapter-cloudflare/index.js

@@ -1,4 +1,4 @@
-import { existsSync, writeFileSync } from 'node:fs';
+import { existsSync, readFileSync, writeFileSync } from 'node:fs';
 import * as path from 'node:path';
 import { fileURLToPath } from 'node:url';
 import * as esbuild from 'esbuild';
@@ -143,6 +143,20 @@ export default function (options = {}) {
 					}`
 				);
 			}
+
+			const assetsignore = [`${dest}/.assetsignore`, `${files}/.assetsignore`].reduce(
+				(acc, file) => {
+					if (existsSync(file)) {
+						readFileSync(file, 'utf8')
+							.split('\n')
+							.filter((line) => line.trim())
+							.forEach((line) => acc.add(line));
+					}
+					return acc;
+				},
+				new Set()
+			);
+			writeFileSync(`${dest}/.assetsignore`, Array.from(assetsignore).sort().join('\n') + '\n');
 		},
 		async emulate() {
 			const proxy = await getPlatformProxy(options.platformProxy);