希望支持出码方案的切换

```
import * as CodeGenerator from '@alilc/lowcode-code-generator/standalone-loader';
const result = await CodeGenerator.generateCode({
solution: 'icejs', // 出码方案 (目前内置有 icejs 和 rax )
schema, // 编排搭建出来的 schema
});
```

官方给的示例代码里, solution只支持内置的2种, 开发者自定义的出码方案无法集成

Start with the standalone-loader entry point and the generateCode API shown in the issue, then trace how the solution option selects the built-in icejs and rax schemes. Determine the integration boundary for developer-defined code-generation schemes; done means a custom scheme can be selected and used alongside the built-ins.

Written by the indexing model from the issue text.