Optimize web-js to be usable also in MPAs (multi page applications, e.g. PHP)

[incorbador]

Why

Initially, we developed web-js to be used by SPAs primarily (we didn't explicitly exclude MPAs, but we also never really tested them much). After working on #166 we should now make sure that vanilla-js can also be used with MPAs by creating one demo PHP application.

TODOS

• create a PHP demo application
• figure out, if we can deploy it to Vercel (if possible, do it)
• make adaptions to web-js (and the other packages) if needed