Allow to pass signals to borrowed to sizes

[mondeja]

Currently, if you try to pass a Signal<&str> to Icon's props width and height, the next error is raised:

the trait `From<leptos::prelude::Signal<&str>>` is not implemented for `MaybeProp<std::string::String>`

This is not happening for plain &'static str because there is a impl for Debug here:

impl<T, S> Debug for MaybeProp<T, S>
where
    T: Debug + 'static,

So better use TextProp.

[mondeja]

Is not unwind safe because the calls to .get() are propagating to into_attribute_value.

https://github.com/leptos-rs/leptos/blob/f51c676e0df12a0b1192ec599360ce8006eb9750/leptos/src/text_prop.rs#L78

impl IntoAttributeValue for TextProp {
    type Output = Arc<dyn Fn() -> Oco<'static, str> + Send + Sync>;

    fn into_attribute_value(self) -> Self::Output {
        self.0
    }
}