workspace: Add setting to make dock resize apply to all panels (#30551)

Re: #19015
Close #12667

When dragging a dock’s resize handle, only the active panel grows or
shrinks. This patch introduces an opt-in behaviour that lets users
resize every panel hosted by that dock at once.

Release Notes:

- Added new `resize_all_panels_in_dock` setting to optionally resize
every panel in a dock together.

Co-authored-by: Mikayla Maki <mikayla@zed.dev>

crates/workspace/src/workspace_settings.rs

@@ -1,5 +1,6 @@
 use std::num::NonZeroUsize;
 
+use crate::DockPosition;
 use anyhow::Result;
 use collections::HashMap;
 use gpui::App;
@@ -26,6 +27,7 @@ pub struct WorkspaceSettings {
     pub max_tabs: Option<NonZeroUsize>,
     pub when_closing_with_no_tabs: CloseWindowWhenNoItems,
     pub on_last_window_closed: OnLastWindowClosed,
+    pub resize_all_panels_in_dock: Vec<DockPosition>,
     pub close_on_file_delete: bool,
 }
 
@@ -192,6 +194,10 @@ pub struct WorkspaceSettingsContent {
     ///
     /// Default: auto (nothing on macOS, "app quit" otherwise)
     pub on_last_window_closed: Option<OnLastWindowClosed>,
+    /// Whether to resize all the panels in a dock when resizing the dock.
+    ///
+    /// Default: ["left"]
+    pub resize_all_panels_in_dock: Option<Vec<DockPosition>>,
     /// Whether to automatically close files that have been deleted on disk.
     ///
     /// Default: false