debugger: Add actions and keybindings for opening the thread and session menus (#31135)

Makes it possible to open and navigate these menus from the keyboard.

I also removed the eager previewing behavior for the thread picker,
which was buggy and came with a jarring layout shift.

Release Notes:

- Debugger Beta: Added the `debugger: open thread picker` and `debugger:
open session picker` actions.

assets/keymaps/default-linux.json

@@ -862,6 +862,13 @@
       "alt-l": "git::GenerateCommitMessage"
     }
   },
+  {
+    "context": "DebugPanel",
+    "bindings": {
+      "ctrl-t": "debugger::ToggleThreadPicker",
+      "ctrl-i": "debugger::ToggleSessionPicker"
+    }
+  },
   {
     "context": "CollabPanel && not_editing",
     "bindings": {