Fix compiler error in examples/fullstack-auth

[ywiyogo]

Running dx serve --fullstack in fullstack-auth returns several compiler errors:

[cargo] error[E0432]: unresolved import `web_sys::DomRectReadOnly`
  --> packages/web/src/events/visible.rs:11:28
   |
11 | use web_sys::{CustomEvent, DomRectReadOnly, Event, IntersectionObserverEntry};
   |                            ^^^^^^^^^^^^^^^ no `DomRectReadOnly` in the root

This feature activation of web-sys should resolve the issue.