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/actions.rs

@@ -340,7 +340,9 @@ gpui::actions!(
         MoveToPreviousWordStart,
         MoveToStartOfParagraph,
         MoveToStartOfExcerpt,
+        MoveToStartOfNextExcerpt,
         MoveToEndOfExcerpt,
+        MoveToEndOfPreviousExcerpt,
         MoveUp,
         Newline,
         NewlineAbove,
@@ -378,7 +380,9 @@ gpui::actions!(
         SelectAll,
         SelectAllMatches,
         SelectToStartOfExcerpt,
+        SelectToStartOfNextExcerpt,
         SelectToEndOfExcerpt,
+        SelectToEndOfPreviousExcerpt,
         SelectDown,
         SelectEnclosingSymbol,
         SelectLargerSyntaxNode,