Allow extensions to define more of the methods in the LspAdapter trait (#9554)

Our goal is to extract Svelte support into an extension, since we've
seen problems with the Tree-sitter Svelte parser crashing due to bugs in
the external scanner. In order to do this, we need a couple more
capabilities in LSP extensions:

* [x] `initialization_options` - programmatically controlling the JSON
initialization params sent to the language server
* [x] `prettier_plugins` - statically specifying a list of prettier
plugins that apply for a given language.
* [x] `npm_install_package`

Release Notes:

- N/A

---------

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

crates/languages/src/css.rs

@@ -91,10 +91,13 @@ impl LspAdapter for CssLspAdapter {
         get_cached_server_binary(container_dir, &*self.node).await
     }
 
-    fn initialization_options(&self) -> Option<serde_json::Value> {
-        Some(json!({
+    async fn initialization_options(
+        self: Arc<Self>,
+        _: &Arc<dyn LspAdapterDelegate>,
+    ) -> Result<Option<serde_json::Value>> {
+        Ok(Some(json!({
             "provideFormatter": true
-        }))
+        })))
     }
 }