languages: Remove a eager conversion from LanguageName to String (#35667)

This PR changes the signature of `language_names` from

```rust
pub fn language_names(&self) -> Vec<String>
// Into
pub fn language_names(&self) -> Vec<LanguageName>
```

The function previously eagerly converted `LanguageName`'s to
`String`'s, which requires the reallocation of all of the elements. The
functions get called in many places in the code base, but only one of
which actually requires the conversion to a `String`. In one case it
would do a `SharedString` -> `String` -> `SharedString` conversion,
which is now totally bypassed.

Release Notes:

- N/A

crates/language_selector/src/language_selector.rs

@@ -121,13 +121,13 @@ impl LanguageSelectorDelegate {
             .into_iter()
             .filter_map(|name| {
                 language_registry
-                    .available_language_for_name(&name)?
+                    .available_language_for_name(name.as_ref())?
                     .hidden()
                     .not()
                     .then_some(name)
             })
             .enumerate()
-            .map(|(candidate_id, name)| StringMatchCandidate::new(candidate_id, &name))
+            .map(|(candidate_id, name)| StringMatchCandidate::new(candidate_id, name.as_ref()))
             .collect::<Vec<_>>();
 
         Self {