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/extension_api/src/extension_api.rs

@@ -13,6 +13,14 @@ pub trait Extension: Send + Sync {
         config: wit::LanguageServerConfig,
         worktree: &wit::Worktree,
     ) -> Result<Command>;
+
+    fn language_server_initialization_options(
+        &mut self,
+        _config: wit::LanguageServerConfig,
+        _worktree: &wit::Worktree,
+    ) -> Result<Option<String>> {
+        Ok(None)
+    }
 }
 
 #[macro_export]
@@ -60,4 +68,11 @@ impl wit::Guest for Component {
     ) -> Result<wit::Command> {
         extension().language_server_command(config, worktree)
     }
+
+    fn language_server_initialization_options(
+        config: LanguageServerConfig,
+        worktree: &Worktree,
+    ) -> Result<Option<String>, String> {
+        extension().language_server_initialization_options(config, worktree)
+    }
 }