Allow customization of the model used for tool calling (#15479)

We also eliminate the `completion` crate and moved its logic into
`LanguageModelRegistry`.

Release Notes:

- N/A

---------

Co-authored-by: Nathan <nathan@zed.dev>

crates/semantic_index/Cargo.toml

@@ -22,7 +22,6 @@ anyhow.workspace = true
 client.workspace = true
 clock.workspace = true
 collections.workspace = true
-completion.workspace = true
 fs.workspace = true
 futures.workspace = true
 futures-batch.workspace = true

crates/semantic_index/src/semantic_index.rs

@@ -1261,6 +1261,3 @@ mod tests {
         );
     }
 }
-
-// See https://github.com/zed-industries/zed/pull/14823#discussion_r1684616398 for why this is here and when it should be removed.
-type _TODO = completion::LanguageModelCompletionProvider;