[Feature] Reference types of option properties easily

### What problem does this feature solve?

I've found that it is quite hard to use types with ECharts. For instance, I wanted to create a function that was my default tooltip formatter (to reuse the code).

The type it should have is:
```ts
formatter?: string | TooltipFormatterCallback;
```

However, these types are not exported. I tried to pick the type using typescript-->
```
ECOption["tooltip"]["formatter"]
```

But the formatter can either be an array or a single object so typescript doesn't allow me to do this.

Similar things happen with xAxis and others (all the types that are `Arrayable`). I just don't know how to get the types or infer them in some way.

Maybe I'm missing something. If I am maybe, adding a small explanation in the docs could prove helpful.

### What does the proposed API look like?

Export the typings of individual options so that they can be used easily