Allow loading echarts library lazily via native import

[smnbbrv]

Hi @xieziyu

I did not bother you for a long time, it's time to do this again :)

The new way of importing the echarts is very heavy, because it loads the whole echarts in the application right away; however in most of my projects the echarts is a helper library which appears on several secondary pages.

Even if you make a custom build, it's still 170K gzipped which needs to be downloaded and parsed => performance goes down.

This PR aims this issue.

Actually I wanted to use this approach for protobuf Any data type at my https://github.com/ngx-grpc/ngx-grpc for a long time and always didn't have this time to implement it. So, sorry, but looks like we need to test it with ngx-echarts :)

The change is not breaking, and to use all its goodness one just need to change the imports in app.module.ts

NgxEchartsModule.forRoot({ echarts: () => import('./my-custom-echarts') }),

Please attack me if I miss something.

Thanks!

[xieziyu]

Thank you!