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/zed/src/zed.rs

@@ -4439,7 +4439,7 @@ mod tests {
         });
         for name in languages.language_names() {
             languages
-                .language_for_name(&name)
+                .language_for_name(name.as_ref())
                 .await
                 .with_context(|| format!("language name {name}"))
                 .unwrap();