Add holding opt/alt for fast scrolling (#31056)

Fixes #14612

This was a feature I dearly missed from VSCode, so adding this helped me
migrate to Zed without disrupting my workflow. I found that `4.0` was a
nice goldilocks multiplier and felt close/the same as the speed in
VSCode.

Release Notes:

- Added faster scrolling in the editor while holding opt/alt

crates/editor/src/editor_settings.rs

@@ -26,6 +26,7 @@ pub struct EditorSettings {
     pub autoscroll_on_clicks: bool,
     pub horizontal_scroll_margin: f32,
     pub scroll_sensitivity: f32,
+    pub fast_scroll_sensitivity: f32,
     pub relative_line_numbers: bool,
     pub seed_search_query_from_cursor: SeedQuerySetting,
     pub use_smartcase_search: bool,
@@ -406,6 +407,12 @@ pub struct EditorSettingsContent {
     ///
     /// Default: 1.0
     pub scroll_sensitivity: Option<f32>,
+    /// Scroll sensitivity multiplier for fast scrolling. This multiplier is applied
+    /// to both the horizontal and vertical delta values while scrolling. Fast scrolling
+    /// happens when a user holds the alt or option key while scrolling.
+    ///
+    /// Default: 4.0
+    pub fast_scroll_sensitivity: Option<f32>,
     /// Whether the line numbers on editors gutter are relative or not.
     ///
     /// Default: false
@@ -745,6 +752,10 @@ impl Settings for EditorSettings {
             "editor.mouseWheelScrollSensitivity",
             &mut current.scroll_sensitivity,
         );
+        vscode.f32_setting(
+            "editor.fastScrollSensitivity",
+            &mut current.fast_scroll_sensitivity,
+        );
         if Some("relative") == vscode.read_string("editor.lineNumbers") {
             current.relative_line_numbers = Some(true);
         }