E2E testing

[NotAShelf]

It seems that one of the weakest aspects of ndg currently is validating the default templates beyond a real human looking at the generating pages, and the rest of us hoping it's enough. As an alternative I'd like to integrate an end-to-end test setup that tests thet entire pipeline. Something like:

inputs (content, config, templates) -> generator binary -> output (HTML, assets) -> HTTP view (browser layer)

Each test should:

1. Create a temporary workspace (mock input)
2. Run ndg to generate output
3. Serve the generated output
4. Query the site like a browser would
5. Verify HTML, links, and metadata

[NotAShelf]

https://www.npmjs.com/package/browser-ui-test might come in handy. Since it's a NPM package we can introduce a package.json at the repository root, and replace Deno's linter with esling. Alternatively we can continue to use Deno, and use this exclusively for testing in CI.