Function mount 

pub fn mount<F>(selector: &str, render_fn: F)
where
    F: FnOnce() -> VirtualNode,

Mounts the given virtual DOM tree to a specific element matched by a CSS selector.

Supported selector syntax:

• "#id" — select by element ID
• ".class" — select by class name (uses the first match)
• "tag" — select by tag name (uses the first match)

Arguments

• &str - A CSS selector string to locate the target element.
• FnOnce() -> VirtualNode + 'static - A closure that returns the virtual DOM tree to render.