Export Val Variants

[bushrat011899]

Objective

• Fixes Add Val helper functions #15937

Solution

• Export the variants of Val in the bevy_ui prelude.
• Updated examples to use the shortened names.

Testing

• Ran CI.

Notes

I'm not totally convinced this is that big of a deal ergonomically. It's pretty easy for the end-user to write use Val::{...}; at the top of their own files if it really helps them. This was a simple enough PR that I just wanted to create this as a way to decide whether #15937 should just be closed.

[ickshonpe]

I don't hate this but I have a strong preference for the free functions alternative. Except for ubiquitous stuff like Option I don't like enum exports, (like you say) users can easily add bevy::Val::* to their own project's prelude if they want to, px is easier to type than Px and a free function API would be more flexible. A px function could support:

pub trait AsValNum {
    fn as_val_num(self) -> f32;
}

impl AsValNum for i32 {
    fn as_val_num(self) -> f32 {
        self as f32
    }
}

impl AsValNum for f32 {
    fn as_val_num(self) -> f32 {
        self
    }
}

pub fn px(n: AsValNum) -> Val {
    Val::Px(n.as_val_num())
}

for instance. Maybe AsValNum (or whatever) would be controversial and I'm not suggesting it has to be included but it's available to us as an option with free functions. I just find it unpleasant that probably 99% of Bevy UI code uses integers but the API has to require floats everywhere.