[Feature] Suggest adding the function of "Generate on-demand import code" on the builder page.

### What problem does this feature solve?

# 建议在定制页面增加“生成按需导入的代码”的功能。

Suggest adding the function of "Generate on-demand import code" on the builder page.

## 动机 Motivation

ECharts 有很多模块和组件，文档又没有很详细的问题说明，想要按需导入，非常麻烦。

ECharts has many modules and components, and the documentation does not provide detailed explanations. It is very troublesome to import on demand.

很多时候往往不知道应该导入哪些模块。

Most of the time, it's often unclear which modules should be imported.

[builder page](https://echarts.apache.org/en/builder.html)

### What does the proposed API look like?

Generate on-demand import code on [builder page](https://echarts.apache.org/en/builder.html)