fix: renames index.js to enhanceApp.js (#226)

lib/prepare.js

@@ -175,7 +175,7 @@ async function resolveOptions (sourceDir) {
       options.notFoundPath = path.resolve(__dirname, 'default-theme/NotFound.vue')
     }
 
-    const themeEnhanceAppPath = path.resolve(themeDir, 'index.js')
+    const themeEnhanceAppPath = path.resolve(themeDir, 'enhanceApp.js')
     if (fs.existsSync(themeEnhanceAppPath)) {
       options.themeEnhanceAppPath = themeEnhanceAppPath
     }