feat: add typings for scratchpad.config.js

[heyMP]

Need to export Typescript types for users who want to use JSDoc typings in scratchpad.config.js.

Example from web-dev-server config.

export default /** @type {import("@web/test-runner").TestRunnerConfig} */ ({
  files: 'test/**/*.test.js',
  nodeResolve: true,
  ...
})