Vaadin 23 compatibility

[javier-godoy]

Even after fixing #7, the styles are not visible in our Vaadin 23 demo: the month calendars are stamped, but the custom styles from test_year-month-calendar.css are not injected.

In my local environment, with an application that uses the addon in development mode, I observe that every fc-month-calendar is stamped too (and the styles are injected):

However, if the same application is compiled in production mode, the month calendars are not stamped

And the following errors are logged in the browser console: