Namedslots support for web_components

[richard-to]

Discussed in #1178

^{Originally posted by jmfk January 7, 2025}
I noticed namedslots isn't implemented for web_components. It is definitely on my wish list for the future.

Since sending UI events via the server just isn't viable implementing things in web_components seems like the best solution.
But just having a single slot is a bit limiting when building UI components. I'm currently building a resizable layout component, and I want to pass sidebar, content and other pane content to it. The component it self would only handle the resize events and render the other components.