Factor out language model selector into its own crate (#21113)

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

crates/assistant/src/assistant.rs

@@ -5,7 +5,6 @@ pub mod assistant_settings;
 mod context;
 pub mod context_store;
 mod inline_assistant;
-mod model_selector;
 mod patch;
 mod prompt_library;
 mod prompts;
@@ -37,7 +36,6 @@ pub(crate) use inline_assistant::*;
 use language_model::{
     LanguageModelId, LanguageModelProviderId, LanguageModelRegistry, LanguageModelResponseMessage,
 };
-pub(crate) use model_selector::*;
 pub use patch::*;
 pub use prompts::PromptBuilder;
 use prompts::PromptLoadingParams;