[addon-a11y] Export types of params

In order to match addon-a11y's options in our automated a11y testing suite, I'd like to use the same types.

Workaround:

```ts
import type {A11yParameters} from '@storybook/addon-a11y/dist/ts3.9/params';
```

Not sure my PR shows the correct way to do this, or if there are other existing workarounds – I'd appreciate some advice!

addons/a11y/src/index.ts

@@ -4,6 +4,7 @@ import dedent from 'ts-dedent';
 
 export { PARAM_KEY } from './constants';
 export * from './highlight';
+export * from './params';
 
 if (module && module.hot && module.hot.decline) {
   module.hot.decline();