Export types for formatter callback params

### What problem does this feature solve?
would allow to define callback formatter with properly typed arguments: for now I have to type ``params`` to ``any``, loosing typing within the formatter function. For instance:

tooltip.formatter = (params: any) => {
// access to whatever in params is allowed
...
return content;
};

### What does the proposed API look like?
As I get it, ``params`` is expected to be of ``FormatterParams`` type, so exporting in within echarts.d.ts would allow something along the line of :

import {FormatterParams} from "echarts";

...
tooltip.formatter = (params: FormatterParams) => {
// compilation error on access to something to defined in FormatterParams
return content;
};