Use ch-width (0) instead of em-width (m) for gutter width calculation (#32548)

Closes #21860

Release Notes:

- Added `ch_width` and `ch_advance` function alongside their `em_*`
counterparts
- Use `ch_*` version to calculate gutter layouts
- Update a stale comment from changes in #31959

The ch units refer to the width of the number `0` whereas em is the
width of `m` and the actual font size (e.g. 16px means 16 px width of
`m`).

This change has no effect for monospaced fonts but can be drastic for
proportional ones as seen below for "Zed Plex Sans" with a
`"min_line_number_width" = 4`.

<img width="726" alt="Screenshot 2025-06-11 at 15 47 35"
src="https://github.com/user-attachments/assets/aa73f4d4-32bc-42cf-a9f6-7e25fee68c9a"
/>

crates/gpui/src/text_system.rs

@@ -209,6 +209,20 @@ impl TextSystem {
         Ok(self.advance(font_id, font_size, 'm')?.width)
     }
 
+    /// Returns the width of an `ch`.
+    ///
+    /// Uses the width of the `0` character in the given font and size.
+    pub fn ch_width(&self, font_id: FontId, font_size: Pixels) -> Result<Pixels> {
+        Ok(self.typographic_bounds(font_id, font_size, '0')?.size.width)
+    }
+
+    /// Returns the advance width of an `ch`.
+    ///
+    /// Uses the advance width of the `0` character in the given font and size.
+    pub fn ch_advance(&self, font_id: FontId, font_size: Pixels) -> Result<Pixels> {
+        Ok(self.advance(font_id, font_size, '0')?.width)
+    }
+
     /// Get the number of font size units per 'em square',
     /// Per MDN: "an abstract square whose height is the intended distance between
     /// lines of type in the same type size"