Fixes to excerpt movement actions and bindings + add multibuffer and singleton_buffer key contexts (#26264)

Closes #26002 

Release Notes:

- Added `multibuffer` key context.
- `cmd-down` and `cmd-shift-down` on Mac now moves to the end of the
last line of a singleton buffer instead of the beginning. In
multibuffers, these now move to the start of the next excerpt.
- Fixed `vim::PreviousSectionEnd` (bound to `[ ]`) to move to the
beginning of the line, matching the behavior of `vim::NextSectionEnd`.
- Added `editor::MoveToStartOfNextExcerpt` and
`editor::MoveToEndOfPreviousExcerpt`.

crates/editor/src/movement.rs

@@ -448,7 +448,9 @@ pub fn end_of_excerpt(
             if start.row() > DisplayRow(0) {
                 *start.row_mut() -= 1;
             }
-            map.clip_point(start, Bias::Left)
+            start = map.clip_point(start, Bias::Left);
+            *start.column_mut() = 0;
+            start
         }
         Direction::Next => {
             let mut end = excerpt.end_anchor().to_display_point(&map);