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/Cargo.toml

@@ -50,6 +50,7 @@ indexed_docs.workspace = true
 indoc.workspace = true
 language.workspace = true
 language_model.workspace = true
+language_model_selector.workspace = true
 language_models.workspace = true
 log.workspace = true
 lsp.workspace = true