Switch to columnar selection by pressing alt-shift while mouse is down (#25096)

We begin a columnar selection when we drag the mouse while holding
`alt-shift`. This PR makes it possible to start the selection and then
turn it into columnar by pressing `alt-shift`.

Fixes #5372 

Release Notes:

- Support switching to columnar selection by pressing `alt-shift` while
mouse is down

crates/editor/src/element.rs

@@ -21,8 +21,9 @@ use crate::{
     GutterDimensions, HalfPageDown, HalfPageUp, HandleInput, HoveredCursor, InlineCompletion,
     JumpData, LineDown, LineUp, OpenExcerpts, PageDown, PageUp, Point, RevertSelectedHunks, RowExt,
     RowRangeExt, SelectPhase, SelectedTextHighlight, Selection, SoftWrap, StickyHeaderExcerpt,
-    ToPoint, ToggleFold, ToggleStagedSelectedDiffHunks, CURSORS_VISIBLE_FOR, FILE_HEADER_HEIGHT,
-    GIT_BLAME_MAX_AUTHOR_CHARS_DISPLAYED, MAX_LINE_LEN, MULTI_BUFFER_EXCERPT_HEADER_HEIGHT,
+    ToPoint, ToggleFold, ToggleStagedSelectedDiffHunks, COLUMNAR_SELECTION_MODIFIERS,
+    CURSORS_VISIBLE_FOR, FILE_HEADER_HEIGHT, GIT_BLAME_MAX_AUTHOR_CHARS_DISPLAYED, MAX_LINE_LEN,
+    MULTI_BUFFER_EXCERPT_HEADER_HEIGHT,
 };
 use buffer_diff::{DiffHunkSecondaryStatus, DiffHunkStatus};
 use client::ParticipantIndex;
@@ -515,6 +516,7 @@ impl EditorElement {
                     if editor.hover_state.focused(window, cx) {
                         return;
                     }
+
                     editor.handle_modifiers_changed(event.modifiers, &position_map, window, cx);
                 })
             }
@@ -594,7 +596,7 @@ impl EditorElement {
 
         let point_for_position = position_map.point_for_position(event.position);
         let position = point_for_position.previous_valid;
-        if modifiers.shift && modifiers.alt {
+        if modifiers == COLUMNAR_SELECTION_MODIFIERS {
             editor.select(
                 SelectPhase::BeginColumnar {
                     position,