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.

Cargo.lock

@@ -230,6 +230,7 @@ dependencies = [
  "schemars",
  "serde",
  "serde_json",
+ "strum",
  "tokio",
 ]
 
@@ -376,6 +377,7 @@ dependencies = [
  "settings",
  "smol",
  "strsim 0.11.1",
+ "strum",
  "telemetry_events",
  "theme",
  "tiktoken-rs",
@@ -6983,6 +6985,7 @@ dependencies = [
  "schemars",
  "serde",
  "serde_json",
+ "strum",
 ]
 
 [[package]]