language_model: Remove dependency on inline_completion_button (#20930)

This PR removes a dependency on the `inline_completion_button` crate
from the `language_model` crate.

We were taking on this dependency solely to call `initiate_sign_in`,
which can easily be moved to the `copilot` crate.

This allows `language_model` to move up in the crate dependency graph.

Release Notes:

- N/A

crates/copilot/src/copilot.rs

@@ -38,8 +38,8 @@ use std::{
 };
 use util::{fs::remove_matching, maybe, ResultExt};
 
-pub use copilot_completion_provider::CopilotCompletionProvider;
-pub use sign_in::CopilotCodeVerification;
+pub use crate::copilot_completion_provider::CopilotCompletionProvider;
+pub use crate::sign_in::{initiate_sign_in, CopilotCodeVerification};
 
 actions!(
     copilot,