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/php.rs

@@ -97,24 +97,9 @@ impl LspAdapter for IntelephenseLspAdapter {
         get_cached_server_binary(container_dir, &*self.node).await
     }
 
-    async fn label_for_completion(
-        &self,
-        _item: &lsp::CompletionItem,
-        _language: &Arc<language::Language>,
-    ) -> Option<language::CodeLabel> {
-        None
-    }
-
-    fn initialization_options(&self) -> Option<serde_json::Value> {
-        None
-    }
     fn language_ids(&self) -> HashMap<String, String> {
         HashMap::from_iter([("PHP".into(), "php".into())])
     }
-
-    fn prettier_plugins(&self) -> &[&'static str] {
-        &["@prettier/plugin-php"]
-    }
 }
 
 async fn get_cached_server_binary(