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/text_system.rs

@@ -33,7 +33,7 @@ pub struct FontId(pub usize);
 #[derive(Hash, PartialEq, Eq, Clone, Copy, Debug)]
 pub struct FontFamilyId(pub usize);
 
-pub const SUBPIXEL_VARIANTS: u8 = 4;
+pub(crate) const SUBPIXEL_VARIANTS: u8 = 4;
 
 pub struct TextSystem {
     line_layout_cache: Arc<LineLayoutCache>,