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

assets/settings/default.json

@@ -475,6 +475,10 @@
   // Scroll sensitivity multiplier. This multiplier is applied
   // to both the horizontal and vertical delta values while scrolling.
   "scroll_sensitivity": 1.0,
+  // 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.
+  "fast_scroll_sensitivity": 4.0,
   "relative_line_numbers": false,
   // If 'search_wrap' is disabled, search result do not wrap around the end of the file.
   "search_wrap": true,