Read svelte.config.js (#428)

Partially reverts #382

The reason given there was that:
> which may not be what storybook users want / expect, since their production config may be different from the storybook config (same reason we don't automatically merge vite.config.js)

However, I think that users would expect their `svelte.config.js` to be taken into account. E.g. right now the user's preprocessing configuration is ignored. The user can still pass custom `svelteOptions` in `.storybook/main.cjs` if they need to override their normal options for some reason (although I can't think of a reason off the top of my head why this would be necessary)

You can see a number of people complaining preprocessing doesn't work here: storybookjs/addon-svelte-csf#4

examples/svelte/.storybook/main.ts

@@ -20,7 +20,6 @@ export default withOverview(__dirname)({
   },
   // @ts-ignore
   svelteOptions: {
-    preprocess: preprocess(),
     // Possible with @sveltejs/vite-plugin-svelte version 1.0.0-next.43 or higher.
     // Focus a story iframe and press cmd+shift (mac) or ctrl+shift (windows) to activate.
     experimental: { inspector: true },

package.json

@@ -47,6 +47,7 @@
     "typescript": "^4.5.4"
   },
   "dependencies": {
+    "@sveltejs/vite-plugin-svelte": "^1.0.0-next.49",
     "bookcase-builder": "^0.0.8"
   },
   "bookcase-builder": {

packages/builder-vite/vite-config.ts

@@ -2,6 +2,7 @@ import * as path from 'path';
 import fs from 'fs';
 import { Plugin } from 'vite';
 import { TypescriptConfig } from '@storybook/core-common';
+import { loadSvelteConfig } from '@sveltejs/vite-plugin-svelte';
 import viteReact from '@vitejs/plugin-react';
 
 import { allowedEnvPrefix as envPrefix } from './envs';
@@ -119,10 +120,9 @@ export async function pluginConfig(options: ExtendedOptions, _type: PluginConfig
       // Non-story svelte files
       // Starting in 1.0.0-next.42, svelte.config.js is included by default.
       // We disable that, but allow it to be overridden in svelteOptions
-      plugins.push(sveltePlugin({ configFile: false, ...svelteOptions, exclude: [...userExclude, ...storyPatterns] }));
+      plugins.push(sveltePlugin({ ...svelteOptions, exclude: [...userExclude, ...storyPatterns] }));
       // Svelte stories without HMR
       const storySveltePlugin = sveltePlugin({
-        configFile: false,
         ...svelteOptions,
         exclude: userExclude,
         include: storyPatterns,
@@ -146,7 +146,8 @@ export async function pluginConfig(options: ExtendedOptions, _type: PluginConfig
 
     try {
       const csfPlugin = require('./svelte/csf-plugin').default;
-      plugins.push(csfPlugin(svelteOptions));
+      const config = loadSvelteConfig();
+      plugins.push(csfPlugin({ ...config, ...svelteOptions }));
     } catch (err) {
       // Not all projects use `.stories.svelte` for stories, and by default 6.5+ does not auto-install @storybook/addon-svelte-csf.
       // If it's any other kind of error, re-throw.

yarn.lock

@@ -4273,6 +4273,27 @@ __metadata:
   languageName: node
   linkType: hard
 
+"@sveltejs/vite-plugin-svelte@npm:^1.0.0-next.49":
+  version: 1.0.0-next.49
+  resolution: "@sveltejs/vite-plugin-svelte@npm:1.0.0-next.49"
+  dependencies:
+    "@rollup/pluginutils": ^4.2.1
+    debug: ^4.3.4
+    deepmerge: ^4.2.2
+    kleur: ^4.1.4
+    magic-string: ^0.26.2
+    svelte-hmr: ^0.14.12
+  peerDependencies:
+    diff-match-patch: ^1.0.5
+    svelte: ^3.44.0
+    vite: ^2.9.0
+  peerDependenciesMeta:
+    diff-match-patch:
+      optional: true
+  checksum: d87d5a3762eb6bd3970b17d9405537b080a46a2301c487faf6efb776f6102fd1a030fd67d06e88a2cbabadc9162410467cc2dc52624a6edc9ed8cdefe60e635d
+  languageName: node
+  linkType: hard
+
 "@tootallnate/once@npm:1":
   version: 1.1.2
   resolution: "@tootallnate/once@npm:1.1.2"
@@ -13843,6 +13864,15 @@ __metadata:
   languageName: node
   linkType: hard
 
+"magic-string@npm:^0.26.2":
+  version: 0.26.2
+  resolution: "magic-string@npm:0.26.2"
+  dependencies:
+    sourcemap-codec: ^1.4.8
+  checksum: b4db4e2b370ac8d9ffc6443a2b591b75364bf1fc9121b5a4068d5b89804abff6709d1fa4a0e0c2d54f2e61e0e44db83efdfe219a5ab0ba6d25ee1f2b51fbed55
+  languageName: node
+  linkType: hard
+
 "make-dir@npm:^2.0.0, make-dir@npm:^2.1.0":
   version: 2.1.0
   resolution: "make-dir@npm:2.1.0"
@@ -18475,6 +18505,7 @@ __metadata:
   version: 0.0.0-use.local
   resolution: "storybook-builder-vite-parent@workspace:."
   dependencies:
+    "@sveltejs/vite-plugin-svelte": ^1.0.0-next.49
     "@typescript-eslint/eslint-plugin": ^5.9.0
     "@typescript-eslint/parser": ^5.9.0
     bookcase-builder: ^0.0.8
@@ -18846,6 +18877,15 @@ __metadata:
   languageName: node
   linkType: hard
 
+"svelte-hmr@npm:^0.14.12":
+  version: 0.14.12
+  resolution: "svelte-hmr@npm:0.14.12"
+  peerDependencies:
+    svelte: ">=3.19.0"
+  checksum: ac5f91030007c709e92992e9a06cda59304fe99c428d163aeff11c005b4bb293c504f1d1aa4a173cf27bbe38eaf7544956299074402efa1abb5ef61573b3f408
+  languageName: node
+  linkType: hard
+
 "svelte-preprocess@npm:^4.10.4":
   version: 4.10.6
   resolution: "svelte-preprocess@npm:4.10.6"