For republishing.

Use new URLs for publishing.

We have added a more meaningful code sample in the README file.

Also, the publishing will happen on multiple registries, just like
`@theoryofnekomata/react-musical-keyboard`

Attempt to unify tests for Jest and Cypress.

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.