[docs] Use jsdoc types in config examples

[LarsDenBakker]

When documenting config examples, we should use jsdocs to add types for the config. This helps a lot with discoverability of config options.

Example:

//@ts-check
import { playwrightLauncher } from '@web/test-runner-playwright';

export default /** @type {import('@web/test-runner').TestRunnerConfig} */ ({
  files: 'demo/test/pass-*.test.js',
  groups: [
    {
      name: 'group-a',
      browsers: [playwrightLauncher({ product: 'firefox' })],
    },
  ],
});