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

docs/src/configuring_zed.md

@@ -142,6 +142,24 @@ For example, to disable ligatures for a given font you can add the following to
 
 `boolean` values
 
+## Centered Layout
+
+- Description: Configuration for the centered layout mode.
+- Setting: `centered_layout`
+- Default:
+
+```json
+"centered_layout": {
+  "left_padding": 0.2,
+  "right_padding": 0.2,
+}
+```
+
+**Options**
+
+The `left_padding` and `right_padding` options define the relative width of the
+left and right padding of the central pane from the workspace when the centered layout mode is activated. Valid values range is from `0` to `0.45`.
+
 ## Copilot
 
 - Description: Copilot-specific settings.