Add editor::ToggleFoldAll action (#34317)

In multibuffers adds the ability to alt-click to fold/unfold all
excepts. In singleton buffers it adds the ability to toggle back and
forth between `editor::FoldAll` and `editor::UnfoldAll`.

Bind it in your keymap with:

```json
  {
    "context": "Editor && (mode == full || multibuffer)",
    "bindings": {
      "cmd-k cmd-o": "editor::ToggleFoldAll"
    }
  },
```

<img width="253" height="99" alt="Screenshot 2025-07-11 at 17 04 25"
src="https://github.com/user-attachments/assets/94de8275-d2ee-4cf8-a46c-a698ccdb60e3"
/>

Release Notes:

- Add ability to fold all excerpts in a multibuffer (alt-click) and in
singleton buffers `editor::ToggleFoldAll`

crates/editor/src/actions.rs

@@ -410,6 +410,8 @@ actions!(
         ToggleFold,
         /// Toggles recursive folding at the current position.
         ToggleFoldRecursive,
+        /// Toggles all folds in a buffer or all excerpts in multibuffer.
+        ToggleFoldAll,
         /// Formats the entire document.
         Format,
         /// Formats only the selected text.