Add editor actions for moving and selecting to next / previous excerpt (#25299)

Covers part of #5129 by adding `MoveToStartOfExcerpt`,
`MoveToEndOfExcerpt`, `SelectToStartOfExcerpt`, and
`SelectToEndOfExcerpt`.

No default linux bindings yet as it's unclear what to use. Currently,
`ctrl-up` / `ctrl-down` scroll up and down by one line (see #13269).
Considering changing the meaning of those.

Mac:

* Previously `cmd-up` and `cmd-down` were `editor::MoveToBeginning` and
`editor::MoveToEnd`. In singleton editors these will behave the same as
before. In multibuffers, they will now step through excerpts instead of
jumping to the beginning / end of the multibuffer.

* `cmd-home` and `cmd-end`, often typed as `cmd-fn-left` and
`cmd-fn-right` are now `editor::MoveToBeginning` and
`editor::MoveToEnd`. This is useful in multibuffers.

Release Notes:

- Mac: `cmd-up` now moves to the previous
multibuffer excerpt start, and `cmd-down` moves to the next multibuffer
excerpt end. Within normal buffers these behave the same as before, moving
to the beginning or end.

crates/editor/src/movement.rs

@@ -7,6 +7,7 @@ use gpui::{Pixels, WindowTextSystem};
 use language::Point;
 use multi_buffer::{MultiBufferRow, MultiBufferSnapshot};
 use serde::Deserialize;
+use workspace::searchable::Direction;
 
 use std::{ops::Range, sync::Arc};
 
@@ -403,6 +404,69 @@ pub fn end_of_paragraph(
     map.max_point()
 }
 
+pub fn start_of_excerpt(
+    map: &DisplaySnapshot,
+    display_point: DisplayPoint,
+    direction: Direction,
+) -> DisplayPoint {
+    let point = map.display_point_to_point(display_point, Bias::Left);
+    let Some(excerpt) = map.buffer_snapshot.excerpt_containing(point..point) else {
+        return display_point;
+    };
+    match direction {
+        Direction::Prev => {
+            let mut start = excerpt.start_anchor().to_display_point(&map);
+            if start >= display_point && start.row() > DisplayRow(0) {
+                let Some(excerpt) = map.buffer_snapshot.excerpt_before(excerpt.id()) else {
+                    return display_point;
+                };
+                start = excerpt.start_anchor().to_display_point(&map);
+            }
+            start
+        }
+        Direction::Next => {
+            let mut end = excerpt.end_anchor().to_display_point(&map);
+            *end.row_mut() += 1;
+            map.clip_point(end, Bias::Right)
+        }
+    }
+}
+
+pub fn end_of_excerpt(
+    map: &DisplaySnapshot,
+    display_point: DisplayPoint,
+    direction: Direction,
+) -> DisplayPoint {
+    let point = map.display_point_to_point(display_point, Bias::Left);
+    let Some(excerpt) = map.buffer_snapshot.excerpt_containing(point..point) else {
+        return display_point;
+    };
+    match direction {
+        Direction::Prev => {
+            let mut start = excerpt.start_anchor().to_display_point(&map);
+            if start.row() > DisplayRow(0) {
+                *start.row_mut() -= 1;
+            }
+            map.clip_point(start, Bias::Left)
+        }
+        Direction::Next => {
+            let mut end = excerpt.end_anchor().to_display_point(&map);
+            *end.column_mut() = 0;
+            if end <= display_point {
+                *end.row_mut() += 1;
+                let point_end = map.display_point_to_point(end, Bias::Right);
+                let Some(excerpt) = map.buffer_snapshot.excerpt_containing(point_end..point_end)
+                else {
+                    return display_point;
+                };
+                end = excerpt.end_anchor().to_display_point(&map);
+                *end.column_mut() = 0;
+            }
+            end
+        }
+    }
+}
+
 /// Scans for a boundary preceding the given start point `from` until a boundary is found,
 /// indicated by the given predicate returning true.
 /// The predicate is called with the character to the left and right of the candidate boundary location.