Extension refactor (#20305)

This contains the main changes to the extensions crate from #20049. The
primary goal here is removing dependencies that we can't include on the
remote.


Release Notes:

- N/A

---------

Co-authored-by: Mikayla <mikayla@zed.dev>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>
Co-authored-by: Marshall <marshall@zed.dev>

crates/language/src/language_registry.rs

@@ -288,14 +288,14 @@ impl LanguageRegistry {
             config.name.clone(),
             config.grammar.clone(),
             config.matcher.clone(),
-            move || {
+            Arc::new(move || {
                 Ok(LoadedLanguage {
                     config: config.clone(),
                     queries: Default::default(),
                     toolchain_provider: None,
                     context_provider: None,
                 })
-            },
+            }),
         )
     }
 
@@ -436,9 +436,8 @@ impl LanguageRegistry {
         name: LanguageName,
         grammar_name: Option<Arc<str>>,
         matcher: LanguageMatcher,
-        load: impl Fn() -> Result<LoadedLanguage> + 'static + Send + Sync,
+        load: Arc<dyn Fn() -> Result<LoadedLanguage> + 'static + Send + Sync>,
     ) {
-        let load = Arc::new(load);
         let state = &mut *self.state.write();
 
         for existing_language in &mut state.available_languages {