Export the Viz type

[mdaines]

This adds an export for the Viz type to the declaration file for the viz package. The class itself isn't exported in JavaScript, but TypeScript users may want to refer to the type when dealing with instances of the class.

For example, a wrapper function that accepts an instance of Viz:

import { type Viz } from "@viz-js/viz";

export function myRender(viz: Viz, src: string): string {
  return viz.renderString(src, { graphAttributes: { label: "My graph" } });
}