[feat] add prerender subfolders setting

Usage:
```json
kit: {
    prerender: {
      subfolders: false,
    },
```
Setting the kit.prerender.subfolders setting to false (default is true) will change the filename generation from "/about/index.html" to "/about.html"

Inspiration for the `subfolders` name came from nuxt:
https://nuxtjs.org/docs/configuration-glossary/configuration-generate/#subfolders

- Fixes sveltejs#1443,
- Related to sveltejs#2276, [sapper/sveltejs#1021](sveltejs/sapper#1021)

documentation/docs/14-configuration.md

@@ -43,6 +43,7 @@ const config = {
 		prerender: {
 			crawl: true,
 			enabled: true,
+			subfolders: true,
 			entries: ['*'],
 			onError: 'fail'
 		},
@@ -163,6 +164,7 @@ See [Prerendering](#ssr-and-javascript-prerender). An object containing zero or
 - `crawl` — determines whether SvelteKit should find pages to prerender by following links from the seed page(s)
 - `enabled` — set to `false` to disable prerendering altogether
 - `entries` — an array of pages to prerender, or start crawling from (if `crawl: true`). The `*` string includes all non-dynamic routes (i.e. pages with no `[parameters]` )
+- `subfolders` - set to `false` to disable subfolders for routes: instead of `about/index.html` render `about.html`
 - `onError`
 
   - `'fail'` — (default) fails the build when a routing error is encountered when following a link

packages/kit/src/core/adapt/prerender.js

@@ -138,6 +138,25 @@ export async function prerender({ cwd, out, log, config, build_data, fallback, a
 		return path;
 	}
 
+	/**
+	 * @param {string} path
+	 * @param {boolean} is_html
+	 */
+	function output_filename(path, is_html) {
+		if (path === '/') {
+			return '/index.html';
+		}
+		const parts = path.split('/');
+		if (is_html && parts[parts.length - 1] !== 'index.html') {
+			if (config.kit.prerender.subfolders) {
+				parts.push('index.html');
+			} else {
+				parts[parts.length - 1] += '.html';
+			}
+		}
+		return parts.join('/');
+	}
+
 	/**
 	 * @param {string} decoded_path
 	 * @param {string?} referrer
@@ -174,12 +193,7 @@ export async function prerender({ cwd, out, log, config, build_data, fallback, a
 			const type = headers && headers['content-type'];
 			const is_html = response_type === REDIRECT || type === 'text/html';
 
-			const parts = decoded_path.split('/');
-			if (is_html && parts[parts.length - 1] !== 'index.html') {
-				parts.push('index.html');
-			}
-
-			const file = `${out}${parts.join('/')}`;
+			const file = `${out}${output_filename(decoded_path, is_html)}`;
 			mkdirp(dirname(file));
 
 			if (response_type === REDIRECT) {
@@ -207,12 +221,7 @@ export async function prerender({ cwd, out, log, config, build_data, fallback, a
 
 				const is_html = result.headers['content-type'] === 'text/html';
 
-				const parts = dependency_path.split('/');
-				if (is_html && parts[parts.length - 1] !== 'index.html') {
-					parts.push('index.html');
-				}
-
-				const file = `${out}${parts.join('/')}`;
+				const file = `${out}${output_filename(dependency_path, is_html)}`;
 				mkdirp(dirname(file));
 
 				if (result.body) writeFileSync(file, result.body);

packages/kit/src/core/adapt/test/index.js

@@ -92,6 +92,7 @@ suite('prerender', async () => {
 			appDir: '_app',
 			prerender: {
 				enabled: true,
+				subfolders: true,
 				entries: ['*']
 			}
 		}

packages/kit/src/core/config/index.spec.js

@@ -44,6 +44,7 @@ test('fills in defaults', () => {
 			prerender: {
 				crawl: true,
 				enabled: true,
+				subfolders: true,
 				entries: ['*'],
 				force: undefined,
 				onError: 'fail',
@@ -144,6 +145,7 @@ test('fills in partial blanks', () => {
 			prerender: {
 				crawl: true,
 				enabled: true,
+				subfolders: true,
 				entries: ['*'],
 				force: undefined,
 				onError: 'fail',

packages/kit/src/core/config/options.js

@@ -117,6 +117,7 @@ const options = object(
 			prerender: object({
 				crawl: boolean(true),
 				enabled: boolean(true),
+				subfolders: boolean(true),
 				entries: validate(['*'], (input, keypath) => {
 					if (!Array.isArray(input) || !input.every((page) => typeof page === 'string')) {
 						throw new Error(`${keypath} must be an array of strings`);

packages/kit/src/core/config/test/index.js

@@ -46,6 +46,7 @@ async function testLoadDefaultConfig(path) {
 			prerender: {
 				crawl: true,
 				enabled: true,
+				subfolders: true,
 				entries: ['*'],
 				force: undefined,
 				onError: 'fail',

packages/kit/types/config.d.ts

@@ -61,6 +61,7 @@ export interface Config {
 		prerender?: {
 			crawl?: boolean;
 			enabled?: boolean;
+			subfolders?: boolean;
 			entries?: string[];
 			onError?: PrerenderOnErrorValue;
 		};