fix(create-app): add --open flag

Author: antfu

packages/create-app/template/package.json

@@ -1,7 +1,7 @@
 {
   "private": true,
   "scripts": {
-    "dev": "slidev",
+    "dev": "slidev --open",
     "build": "slidev build",
     "export": "slidev export"
   },

packages/create-theme/template/package.json

@@ -4,7 +4,7 @@
     "slidev"
   ],
   "scripts": {
-    "dev": "slidev example.md",
+    "dev": "slidev example.md --open",
     "build": "slidev build example.md",
     "export": "slidev export example.md",
     "screenshot": "slidev export example.md --format png"