vim: Support za (#18421)

Closes #6822
Updates #5142 

Release Notes:

- Added new fold actions to toggle folds (`cmd-k cmd-l`), fold every
fold (`cmd-k cmd-0`) unfold every fold (`cmd-k cmd-j`) to fold
recursively (`cmd-k cmd-[`) and unfold recursively (`cmd-k cmd-]`).
- vim: Added `za` to toggle fold under cursor.
- vim: Added `zO`/`zC`/`zA` to open, close and toggle folds recursively
(and fixed `zc` to not recurse into selections).
- vim: Added `zR`/`zM` to open/close all folds in the buffer.

assets/keymaps/default-linux.json

@@ -310,6 +310,11 @@
       "ctrl-shift-\\": "editor::MoveToEnclosingBracket",
       "ctrl-shift-[": "editor::Fold",
       "ctrl-shift-]": "editor::UnfoldLines",
+      "ctrl-k ctrl-l": "editor::ToggleFold",
+      "ctrl-k ctrl-[": "editor::FoldRecursive",
+      "ctrl-k ctrl-]": "editor::UnfoldRecursive",
+      "ctrl-k ctrl-0": "editor::FoldAll",
+      "ctrl-k ctrl-j": "editor::UnfoldAll",
       "ctrl-space": "editor::ShowCompletions",
       "ctrl-.": "editor::ToggleCodeActions",
       "alt-ctrl-r": "editor::RevealInFileManager",