Add `html_attrs` macro

[DanielleHuisman]

Add a html_attrs macro that expands to the usual Yew html macro, but with support for dynamic attributes. This would greatly improve the syntax and user experience.

For example:

html_attrs! {
    <Button attr:disabled=false attr:onclick={on_click}>Toggle</Button>
}

Which would expand to:

html! {
    <Button attrs={attrs! { disabled=false onclick={on_click} }}>Toggle</Button>
}

I'm not sure if this is feasible, but it would be a way nicer syntax (similar to Leptos).