gpui: Sweep through cargo doc output and mark dubious items as non-public (#3932)

I essentially went through the publicly exported items and marked these
that are e.g. leaky reexports as pub(crate). I expect that'd be done on
Tuesday anyways.

Release Notes:
- N/A

crates/gpui/src/gpui.rs

@@ -47,7 +47,7 @@ pub use element::*;
 pub use elements::*;
 pub use executor::*;
 pub use geometry::*;
-pub use gpui_macros::*;
+pub use gpui_macros::{register_action, test, IntoElement, Render};
 pub use image_cache::*;
 pub use input::*;
 pub use interactive::*;