Enable clippy::iter_kv_map (#8832)

This PR enables the
[`clippy::iter_kv_map`](https://rust-lang.github.io/rust-clippy/master/index.html#/iter_kv_map)
rule and fixes the outstanding violations.

Release Notes:

- N/A

crates/extension/src/extension_store.rs

@@ -798,8 +798,8 @@ impl ExtensionStore {
                 },
                 grammars: manifest_json
                     .grammars
-                    .into_iter()
-                    .map(|(grammar_name, _)| (grammar_name, Default::default()))
+                    .into_keys()
+                    .map(|grammar_name| (grammar_name, Default::default()))
                     .collect(),
                 language_servers: Default::default(),
             };