This PR begins the process of breaking up the `style_helpers!` macro
into smaller macros that can be used to generate methods for a related
subset of styles.

The style method macros also now accept an optional `visibility`
parameter to control the visibility of the generated methods. This
allows for adding these methods to a struct instead of a just a trait.

For example, to expose just the padding styles on a `Facepile` we can do
this:

```rs
impl Facepile {
    fn style(&mut self) -> &mut StyleRefinement {
        self.base.style()
    }

    gpui::padding_style_methods!({
        visibility: pub
    });
}
```

Release Notes:

- N/A