Added optional cleanup support to onmounted

[rapporian]

Hi @ealmloff, This PR implements the closure strategy discussed. It adds an optional cleanup closure to onmounted handlers. When an element is removed from the DOM, any cleanup closure returned by its onmounted handler is automatically invoked.

Here is how it would be used:

div {
    onmounted: move |e| {
        let el = e.data();
        start_animation(el.clone());
        move || stop_animation(el)
    },
}

Changes

• onmounted handlers can now return a FnOnce() cleanup closure
• Cleanup runs before element removal (remove, replace_with)
• Existing handlers returning () or async {} continue to work
• Web renderer only (desktop/mobile not yet implemented)

Testing

• Added Playwright test that toggles an element and verifies cleanup is called

Documentation

• Documentation PR