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/gpui_macros/src/gpui_macros.rs

@@ -27,13 +27,34 @@ pub fn derive_render(input: TokenStream) -> TokenStream {
     derive_render::derive_render(input)
 }
 
-/// Used by gpui to generate the style helpers.
+/// Used by GPUI to generate the style helpers.
 #[proc_macro]
 #[doc(hidden)]
 pub fn style_helpers(input: TokenStream) -> TokenStream {
     style_helpers::style_helpers(input)
 }
 
+/// Generates methods for margin styles.
+#[proc_macro]
+#[doc(hidden)]
+pub fn margin_style_methods(input: TokenStream) -> TokenStream {
+    style_helpers::margin_style_methods(input)
+}
+
+/// Generates methods for padding styles.
+#[proc_macro]
+#[doc(hidden)]
+pub fn padding_style_methods(input: TokenStream) -> TokenStream {
+    style_helpers::padding_style_methods(input)
+}
+
+/// Generates methods for position styles.
+#[proc_macro]
+#[doc(hidden)]
+pub fn position_style_methods(input: TokenStream) -> TokenStream {
+    style_helpers::position_style_methods(input)
+}
+
 /// #[gpui::test] can be used to annotate test functions that run with GPUI support.
 /// it supports both synchronous and asynchronous tests, and can provide you with
 /// as many `TestAppContext` instances as you need.