Both the JSDOM and the Cypress tests are organized.

This commit adds Cypress for testing the library against a real DOM.

Use custom JSDOM apart from Jest's built-in JSDOM instance because
we need to test different kinds of documents.

We will need to address problems with types, e.g. for `RadioNodeList`
because the runner could not find its declaration (for both our custom
and built-in JSDOM instances).