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/tailwind.rs

@@ -92,8 +92,11 @@ impl LspAdapter for TailwindLspAdapter {
         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,
             "userLanguages": {
                 "html": "html",
@@ -101,7 +104,7 @@ impl LspAdapter for TailwindLspAdapter {
                 "javascript": "javascript",
                 "typescriptreact": "typescriptreact",
             },
-        }))
+        })))
     }
 
     fn workspace_configuration(&self, _workspace_root: &Path, _: &mut AppContext) -> Value {
@@ -126,10 +129,6 @@ impl LspAdapter for TailwindLspAdapter {
             ("PHP".to_string(), "php".to_string()),
         ])
     }
-
-    fn prettier_plugins(&self) -> &[&'static str] {
-        &["prettier-plugin-tailwindcss"]
-    }
 }
 
 async fn get_cached_server_binary(