Fix incorrect assumption about Path.extension() (#24443)

Release Notes:

- N/A

crates/language/src/language_registry.rs

@@ -659,7 +659,10 @@ impl LanguageRegistry {
         user_file_types: Option<&HashMap<Arc<str>, GlobSet>>,
     ) -> Option<AvailableLanguage> {
         let filename = path.file_name().and_then(|name| name.to_str());
-        let extension = path.extension().and_then(|ext| ext.to_str());
+        // `Path.extension()` returns None for files with a leading '.'
+        // and no other extension which is not the desired behavior here,
+        // as we want `.zshrc` to result in extension being `Some("zshrc")`
+        let extension = filename.and_then(|filename| filename.split('.').last());
         let path_suffixes = [extension, filename, path.to_str()];
         let empty = GlobSet::empty();