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/workspace_settings.rs

@@ -7,6 +7,7 @@ use settings::{Settings, SettingsSources};
 #[derive(Deserialize)]
 pub struct WorkspaceSettings {
     pub active_pane_magnification: f32,
+    pub centered_layout: CenteredLayoutSettings,
     pub confirm_quit: bool,
     pub show_call_status_icon: bool,
     pub autosave: AutosaveSetting,
@@ -31,6 +32,8 @@ pub struct WorkspaceSettingsContent {
     ///
     /// Default: `1.0`
     pub active_pane_magnification: Option<f32>,
+    // Centered layout related settings.
+    pub centered_layout: Option<CenteredLayoutSettings>,
     /// Whether or not to prompt the user to confirm before closing the application.
     ///
     /// Default: false
@@ -75,6 +78,21 @@ pub enum AutosaveSetting {
     OnWindowChange,
 }
 
+#[derive(Copy, Clone, Debug, Serialize, Deserialize, JsonSchema)]
+#[serde(rename_all = "snake_case")]
+pub struct CenteredLayoutSettings {
+    /// The relative width of the left padding of the central pane from the
+    /// workspace when the centered layout is used.
+    ///
+    /// Default: 0.2
+    pub left_padding: Option<f32>,
+    // The relative width of the right padding of the central pane from the
+    // workspace when the centered layout is used.
+    ///
+    /// Default: 0.2
+    pub right_padding: Option<f32>,
+}
+
 impl Settings for WorkspaceSettings {
     const KEY: Option<&'static str> = None;