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>

assets/settings/default.json

@@ -533,6 +533,9 @@
       "function": false
     }
   },
+  // Whether to resize all the panels in a dock when resizing the dock.
+  // Can be a combination of "left", "right" and "bottom".
+  "resize_all_panels_in_dock": ["left"],
   "project_panel": {
     // Whether to show the project panel button in the status bar
     "button": true,