Vite throws JSON parsing error when route block has [lang=json]

```vue
<route>
{
  "path": "/"
}
</route>
```
works fine

```vue
<route lang="json">
{
  "path": "/"
}
</route>
```
Vite throws an error about trying to transform malformed JSON.

It's because it's getting JS syntax here:
https://github.com/hannoeru/vite-plugin-pages/blob/main/src/index.ts#L38