Add centered layout support (#9754)

This PR implements the Centered Layout feature (#4685):
- Added the `toggle centered layout` action.
- The centered layout mode only takes effect when there's a single
central pane.
- The state of the centered layout toggle is saved / restored between
Zed restarts.
- The paddings are controlled by the `centered_layout` setting:

```json
"centered_layout": {
  "left_padding": 0.2,
  "right_padding": 0.2
}
```

This allows us to support both the VSCode-style (equal paddings) and
IntelliJ-style (only left padding in Zen mode).

Release Notes:

- Added support for Centered Layout
([#4685](https://github.com/zed-industries/zed/pull/9754)).


https://github.com/zed-industries/zed/assets/2101250/2d5b2a16-c248-48b5-9e8c-6f1219619398

Related Issues:

- Part of #4382

crates/workspace/src/persistence/model.rs

@@ -71,6 +71,7 @@ pub(crate) struct SerializedWorkspace {
     pub(crate) center_group: SerializedPaneGroup,
     pub(crate) bounds: Option<Bounds<DevicePixels>>,
     pub(crate) fullscreen: bool,
+    pub(crate) centered_layout: bool,
     pub(crate) display: Option<Uuid>,
     pub(crate) docks: DockStructure,
 }