editor: Use em_advance everywhere for horizontal scroll position computations (#33514)

Closes #33472

This PR fixes some regressions that were introduced in
https://github.com/zed-industries/zed/pull/32558, which updated the
editor scrolling to use `em_advance` instead of `em_width` for the
horizontal scroll position calculation.
However, not all occurrences were updated, which caused issues with wrap
guides and some small stuttering with horizontal autoscroll whilst
typing/navigating with the keyboard.

Release Notes:

- Fixed an issue where horizontal autoscrolling would stutter and indent
guides would drift when scrolling horizontally.

crates/editor/src/mouse_context_menu.rs

@@ -275,10 +275,10 @@ pub fn deploy_context_menu(
             cx,
         ),
         None => {
-            let character_size = editor.character_size(window);
+            let character_size = editor.character_dimensions(window);
             let menu_position = MenuPosition::PinnedToEditor {
                 source: source_anchor,
-                offset: gpui::point(character_size.width, character_size.height),
+                offset: gpui::point(character_size.em_width, character_size.line_height),
             };
             Some(MouseContextMenu::new(
                 editor,