assistant: Overhaul provider infrastructure (#14929)

<img width="624" alt="image"
src="https://github.com/user-attachments/assets/f492b0bd-14c3-49e2-b2ff-dc78e52b0815">

- [x] Correctly set custom model token count
- [x] How to count tokens for Gemini models?
- [x] Feature flag zed.dev provider
- [x] Figure out how to configure custom models
- [ ] Update docs

Release Notes:

- Added support for quickly switching between multiple language model
providers in the assistant panel

---------

Co-authored-by: Antonio <antonio@zed.dev>

crates/vim/src/vim.rs

@@ -147,7 +147,7 @@ fn register(workspace: &mut Workspace, cx: &mut ViewContext<Workspace>) {
     workspace.register_action(|workspace: &mut Workspace, _: &ToggleVimMode, cx| {
         let fs = workspace.app_state().fs.clone();
         let currently_enabled = VimModeSetting::get_global(cx).0;
-        update_settings_file::<VimModeSetting>(fs, cx, move |setting| {
+        update_settings_file::<VimModeSetting>(fs, cx, move |setting, _| {
             *setting = Some(!currently_enabled)
         })
     });