Remove barrel file from design-system-example (#7580)

### Description

As @lorcan-codes pointed out in #7529, the `design-system` example could
quickly run into performance issues. This was because we were using a
barrel file.

I've swapped the `main` field `./packages/ui/package.json` for an
`exports` field that allows for creating individual entrypoints for the
package to remove this class of issue.

### Testing Instructions

I verified that `turbo build && cd apps/docs && pnpm preview-storybook`
runs correctly. Additionally, I made sure that `turbo dev` works and
that edits to both the `Button` component itself and the story for it
show hot-reloaded changes.

Happy to have someone else check me!


Closes TURBO-2505

examples/design-system/apps/docs/stories/button.stories.tsx

@@ -1,5 +1,5 @@
 import type { Meta, StoryObj } from "@storybook/react";
-import { Button } from "@acme/ui";
+import { Button } from "@acme/ui/button";
 
 const meta: Meta<typeof Button> = {
   component: Button,

examples/design-system/packages/typescript-config/base.json

@@ -9,7 +9,8 @@
     "forceConsistentCasingInFileNames": true,
     "inlineSources": false,
     "isolatedModules": true,
-    "moduleResolution": "node",
+    "module": "ESNext",
+    "moduleResolution": "Bundler",
     "noUnusedLocals": false,
     "noUnusedParameters": false,
     "preserveWatchOutput": true,

examples/design-system/packages/ui/package.json

@@ -1,17 +1,18 @@
 {
   "name": "@acme/ui",
   "version": "0.0.0",
-  "main": "./dist/index.js",
-  "module": "./dist/index.mjs",
-  "types": "./dist/index.d.ts",
   "sideEffects": false,
   "license": "MIT",
-  "files": [
-    "dist/**"
-  ],
+  "exports": {
+    "./button": {
+      "types": "./src/button.tsx",
+      "import": "./dist/button.mjs",
+      "require": "./dist/button.js"
+    }
+  },
   "scripts": {
-    "build": "tsup src/index.tsx --format esm,cjs --dts --external react",
-    "dev": "tsup src/index.tsx --format esm,cjs --watch --dts --external react",
+    "build": "tsup",
+    "dev": "tsup --watch",
     "lint": "eslint . --max-warnings 0",
     "clean": "rm -rf .turbo && rm -rf node_modules && rm -rf dist"
   },

examples/design-system/packages/ui/tsup.config.ts

@@ -0,0 +1,10 @@
+import { defineConfig } from "tsup";
+
+export default defineConfig((options) => ({
+  entryPoints: ["src/button.tsx"],
+  format: ["cjs", "esm"],
+  dts: true,
+  sourcemap: true,
+  external: ["react"],
+  ...options,
+}));

examples/design-system/turbo.json

@@ -5,7 +5,9 @@
       "outputs": ["dist/**", "storybook-static/**"],
       "dependsOn": ["^build"]
     },
-    "lint": {},
+    "lint": {
+      "dependsOn": ["^lint"]
+    },
     "dev": {
       "cache": false,
       "persistent": true