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/platform/mac/text_system.rs

@@ -500,9 +500,9 @@ impl<'a> StringIndexConverter<'a> {
 }
 
 #[repr(C)]
-pub struct __CFTypesetter(c_void);
+pub(crate) struct __CFTypesetter(c_void);
 
-pub type CTTypesetterRef = *const __CFTypesetter;
+type CTTypesetterRef = *const __CFTypesetter;
 
 #[link(name = "CoreText", kind = "framework")]
 extern "C" {