Unify declaration of third-party dependencies

[falkoschindler]

Currently there are three ways to include JavaScript libraries when subclassing UI elements: libraries, exposed_libraries and extra_libraries. This PR experiments with using exposed_libraries only, which seems to work for all elements. A new "signature pad" example demonstrates how to use NPM to define and install JavaScript dependencies in a NiceGUI project.

Open tasks:

• deprecate libraries and extra_libraries parameters?
• replace exposed_libraries with a more intuitive name?
• demonstrate a more complex NPM example, like vue-signature-pad, which has other dependencies? --> No, there's currently no point in trying to integrate third-party Vue components since we're not able to support Vue 3 components at the moment.
• demonstrate a more complex NPM example, like is-odd, using a bundler (see the new "is even" example)
• add some documentation about custom UI elements with third-party dependencies