debugger: Align zoom behavior with other panels (#31901)

Release Notes:

- Debugger Beta: `shift-escape` (`workspace::ToggleZoom`) now zooms the
entire debug panel; `alt-shift-escape` (`debugger::ToggleExpandItem`)
triggers the old behavior of zooming a specific item.

assets/keymaps/default-linux.json

@@ -884,7 +884,8 @@
     "context": "DebugPanel",
     "bindings": {
       "ctrl-t": "debugger::ToggleThreadPicker",
-      "ctrl-i": "debugger::ToggleSessionPicker"
+      "ctrl-i": "debugger::ToggleSessionPicker",
+      "shift-alt-escape": "debugger::ToggleExpandItem"
     }
   },
   {

assets/keymaps/default-macos.json

@@ -949,7 +949,8 @@
     "context": "DebugPanel",
     "bindings": {
       "cmd-t": "debugger::ToggleThreadPicker",
-      "cmd-i": "debugger::ToggleSessionPicker"
+      "cmd-i": "debugger::ToggleSessionPicker",
+      "shift-alt-escape": "debugger::ToggleExpandItem"
     }
   },
   {