gpui: Make style macros more composable (#14007)

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

crates/ui/src/components/facepile.rs

@@ -1,5 +1,5 @@
 use crate::prelude::*;
-use gpui::AnyElement;
+use gpui::{AnyElement, StyleRefinement};
 use smallvec::SmallVec;
 
 /// A facepile is a collection of faces stacked horizontally–
@@ -23,6 +23,23 @@ impl Facepile {
     }
 }
 
+impl ParentElement for Facepile {
+    fn extend(&mut self, elements: impl IntoIterator<Item = AnyElement>) {
+        self.faces.extend(elements);
+    }
+}
+
+// Style methods.
+impl Facepile {
+    fn style(&mut self) -> &mut StyleRefinement {
+        self.base.style()
+    }
+
+    gpui::padding_style_methods!({
+        visibility: pub
+    });
+}
+
 impl RenderOnce for Facepile {
     fn render(self, _cx: &mut WindowContext) -> impl IntoElement {
         // Lay the faces out in reverse so they overlap in the desired order (left to right, front to back)
@@ -40,15 +57,3 @@ impl RenderOnce for Facepile {
             )
     }
 }
-
-impl ParentElement for Facepile {
-    fn extend(&mut self, elements: impl IntoIterator<Item = AnyElement>) {
-        self.faces.extend(elements);
-    }
-}
-
-impl Styled for Facepile {
-    fn style(&mut self) -> &mut gpui::StyleRefinement {
-        self.base.style()
-    }
-}