editor: Push em width calculations down into EditorSnapshot::gutter_dimensions (#24062)

This PR removes the `em_width` and `em_advance` parameters to
`EditorSnapshot::gutter_dimensions` in favor of computing the values
inside of it.

In practice all of the callers were passing in the same values, and
there isn't a circumstance where we would want to pass in different
values.

`gutter_dimensions` has also been modified to return
`Option<GutterDimensions>` instead of `GutterDimensions` so that we can
remove some `.unwrap`s when interacting with the text system.

Release Notes:

- N/A

crates/editor/src/element.rs

@@ -6476,14 +6476,14 @@ impl Element for EditorElement {
 
                     let letter_size = size(em_width, line_height);
 
-                    let gutter_dimensions = snapshot.gutter_dimensions(
-                        font_id,
-                        font_size,
-                        em_width,
-                        em_advance,
-                        self.max_line_number_width(&snapshot, window, cx),
-                        cx,
-                    );
+                    let gutter_dimensions = snapshot
+                        .gutter_dimensions(
+                            font_id,
+                            font_size,
+                            self.max_line_number_width(&snapshot, window, cx),
+                            cx,
+                        )
+                        .unwrap_or_default();
                     let text_width = bounds.size.width - gutter_dimensions.width;
 
                     let editor_width = text_width - gutter_dimensions.margin - em_width;
@@ -8043,17 +8043,11 @@ fn compute_auto_height_layout(
     let font_size = style.text.font_size.to_pixels(window.rem_size());
     let line_height = style.text.line_height_in_pixels(window.rem_size());
     let em_width = window.text_system().em_width(font_id, font_size).unwrap();
-    let em_advance = window.text_system().em_advance(font_id, font_size).unwrap();
 
     let mut snapshot = editor.snapshot(window, cx);
-    let gutter_dimensions = snapshot.gutter_dimensions(
-        font_id,
-        font_size,
-        em_width,
-        em_advance,
-        max_line_number_width,
-        cx,
-    );
+    let gutter_dimensions = snapshot
+        .gutter_dimensions(font_id, font_size, max_line_number_width, cx)
+        .unwrap_or_default();
 
     editor.gutter_dimensions = gutter_dimensions;
     let text_width = width - gutter_dimensions.width;