editor: Add delay for selection drag to prevent accidental drag over attempt for new selection (#32586)

- Add `300ms` delay for it to consider it as selection drag instead of
an attempt to make a new selection.
- Add cursor icon while dragging the selection.

This is same as what chromium does:
https://chromium.googlesource.com/chromium/blink/+/master/Source/core/input/EventHandler.cpp#142

Release Notes:

- Fixed issue where you accidentally end up dragging the selection where
intent was to make a new one instead. To drag selection now, you need to
hold just a little longer before dragging.

crates/editor/src/editor.rs

@@ -912,6 +912,7 @@ enum SelectionDragState {
     ReadyToDrag {
         selection: Selection<Anchor>,
         click_position: gpui::Point<Pixels>,
+        mouse_down_time: Instant,
     },
     /// State when the mouse is dragging the selection in the editor.
     Dragging {