gpui: Add helper methods for em width and em advance (#24036)

This PR adds two helpers methods to the `TextSystem`:

- `em_width`
- `em_advance`

These methods return the width and advance width for an `em`,
respectively.

We were using these definitions in a number of different spots, and by
unifying them we better canonicalize that an `em` is based on the `m`
character.

Release Notes:

- N/A

crates/markdown/src/markdown.rs

@@ -528,12 +528,7 @@ impl MarkdownElement {
         let text_style = self.style.base_text_style.clone();
         let font_id = window.text_system().resolve_font(&text_style.font());
         let font_size = text_style.font_size.to_pixels(window.rem_size());
-        let em_width = window
-            .text_system()
-            .typographic_bounds(font_id, font_size, 'm')
-            .unwrap()
-            .size
-            .width;
+        let em_width = window.text_system().em_width(font_id, font_size).unwrap();
         window.request_autoscroll(Bounds::from_corners(
             point(position.x - 3. * em_width, position.y - 3. * line_height),
             point(position.x + 3. * em_width, position.y + 3. * line_height),