fix: set default node version to 18 in CodeSandbox (#164)

apps/www/.vitepress/theme/utils/codeeditor.ts

@@ -9,6 +9,9 @@ import { type Style } from '@/lib/registry/styles'
 export function makeCodeSandboxParams(componentName: string, style: Style, sources: Record<string, string>) {
   let files = {}
   files = constructFiles(componentName, style, sources)
+  files['.codesandbox/Dockerfile'] = {
+    content: 'FROM node:18',
+  }
   return getParameters({ files, template: 'node' })
 }