Fix block cursor does not render italic for vim (#10249)

Release Notes:

- Fixed #8799

crates/gpui/src/text_system/line_layout.rs

@@ -103,6 +103,18 @@ impl LineLayout {
         self.width
     }
 
+    /// The corresponding Font at the given index
+    pub fn font_id_for_index(&self, index: usize) -> Option<FontId> {
+        for run in &self.runs {
+            for glyph in &run.glyphs {
+                if glyph.index >= index {
+                    return Some(run.font_id);
+                }
+            }
+        }
+        None
+    }
+
     fn compute_wrap_boundaries(
         &self,
         text: &str,