While working on Assistant2, I noticed that the `LanguageModelSelector`
was recreating its `Picker` view on every single render.
This PR makes it so we create the view once and hold onto it in the
parent view.
Release Notes:
- N/A
This PR factors the language model selector out into its own
`language_model_selector` crate so that it can be reused in
`assistant2`.
Also renamed it from `ModelSelector` to `LanguageModelSelector` to be a
bit more specific.
Release Notes:
- N/A