Improve model selection in the assistant (#12472)

https://github.com/zed-industries/zed/assets/482957/3b017850-b7b6-457a-9b2f-324d5533442e


Release Notes:

- Improved the UX for selecting a model in the assistant panel. You can
now switch model using just the keyboard by pressing `alt-m`. Also, when
switching models via the UI, settings will now be updated automatically.

assets/keymaps/default-linux.json

@@ -201,7 +201,8 @@
     "context": "AssistantPanel",
     "bindings": {
       "ctrl-g": "search::SelectNextMatch",
-      "ctrl-shift-g": "search::SelectPrevMatch"
+      "ctrl-shift-g": "search::SelectPrevMatch",
+      "alt-m": "assistant::ToggleModelSelector"
     }
   },
   {

assets/keymaps/default-macos.json

@@ -214,10 +214,11 @@
     }
   },
   {
-    "context": "AssistantPanel", // Used in the assistant crate, which we're replacing
+    "context": "AssistantPanel",
     "bindings": {
       "cmd-g": "search::SelectNextMatch",
-      "cmd-shift-g": "search::SelectPrevMatch"
+      "cmd-shift-g": "search::SelectPrevMatch",
+      "alt-m": "assistant::ToggleModelSelector"
     }
   },
   {