extensions: Add support for snippets provided by extensions (#14020)

For now extensions can only register global snippets, but there'll be
follow-up work to support scope attribute in snippets.json.

Release Notes:

- Extensions can now provide snippets by including `snippets.json` file
next to the extension manifest.

---------

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

crates/zed/src/main.rs

@@ -206,6 +206,7 @@ fn init_ui(app_state: Arc<AppState>, cx: &mut AppContext) -> Result<()> {
     markdown_preview::init(cx);
     welcome::init(cx);
     extensions_ui::init(cx);
+    snippet_provider::init(cx);
 
     // Initialize each completion provider. Settings are used for toggling between them.
     let copilot_language_server_id = app_state.languages.next_language_server_id();