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.

crates/assistant/src/assistant.rs

@@ -2,6 +2,7 @@ pub mod assistant_panel;
 pub mod assistant_settings;
 mod codegen;
 mod completion_provider;
+mod model_selector;
 mod prompts;
 mod saved_conversation;
 mod search;
@@ -15,6 +16,7 @@ use client::{proto, Client};
 use command_palette_hooks::CommandPaletteFilter;
 pub(crate) use completion_provider::*;
 use gpui::{actions, AppContext, Global, SharedString, UpdateGlobal};
+pub(crate) use model_selector::*;
 pub(crate) use saved_conversation::*;
 use semantic_index::{CloudEmbeddingProvider, SemanticIndex};
 use serde::{Deserialize, Serialize};
@@ -38,7 +40,8 @@ actions!(
         InsertActivePrompt,
         ToggleHistory,
         ApplyEdit,
-        ConfirmCommand
+        ConfirmCommand,
+        ToggleModelSelector
     ]
 );