don't make webpack users specify preprocess option

app/svelte/package.json

@@ -54,6 +54,7 @@
     "@storybook/docs-tools": "6.5.0-rc.1",
     "@storybook/node-logger": "6.5.0-rc.1",
     "@storybook/store": "6.5.0-rc.1",
+    "@types/node": "^14.14.20 || ^16.0.0",
     "core-js": "^3.8.2",
     "global": "^4.4.0",
     "loader-utils": "^2.0.0",

app/svelte/src/server/svelte-docgen-loader.ts

@@ -46,7 +46,13 @@ export default async function svelteDocgen(source: string) {
   const { resource } = this._module;
   const svelteOptions: any = { ...getOptions(this) };
 
-  const { preprocess: preprocessOptions, logDocgen = false } = svelteOptions;
+  let configPath = path.join(process.cwd(), './svelte.config.js');
+  if (!fs.existsSync(configPath)) configPath = path.join(process.cwd(), './svelte.config.mjs');
+  if (!fs.existsSync(configPath)) configPath = path.join(process.cwd(), './svelte.config.cjs');
+  if (!fs.existsSync(configPath)) throw new Error('Could not locate Svelte config file');
+  const config = (await import(configPath)).default;
+
+  const { preprocess: preprocessOptions = config.preprocess, logDocgen = false } = svelteOptions;
 
   let docOptions;
   if (preprocessOptions) {

yarn.lock

@@ -8932,6 +8932,7 @@ __metadata:
     "@storybook/node-logger": 6.5.0-rc.1
     "@storybook/store": 6.5.0-rc.1
     "@types/loader-utils": ^2.0.0
+    "@types/node": ^14.14.20 || ^16.0.0
     "@types/webpack-env": ^1.16.0
     core-js: ^3.8.2
     global: ^4.4.0