Attempt to unify tests for Jest and Cypress.

Note on installation.

Include short details on using the library.

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).

Add files from TSDX.