jssdk 多页应用 隐藏菜单

现在是在html用 :root { --Layout-aside-width: 0px; } 隐藏菜单,可否在app 中设定,e.g.

```
{
"type": "app",
"pages": [
{
"url": "/",
"schemaApi": "get:amis/index.json"
}
],
"id": "u:27fa8983c521",
"cssVars":{
"--Layout-aside-width": "0px"
}
}
```

Start by tracing the JSSDK multi-page app configuration and how the app-level cssVars setting is processed. Confirm that setting --Layout-aside-width to 0px in the app configuration hides the menu without requiring a root-level HTML CSS rule, and add coverage for the shown configuration if the project has a relevant test entry point.

Written by the indexing model from the issue text.