Add snippet commands (#18453)

Closes #17860
Closes #15403

Release Notes:

- Added `snippets: configure snippets` command to create and modify
snippets
- Added `snippets: open folder` command for opening the
`~/.config/zed/snippets` directory


https://github.com/user-attachments/assets/fd9e664c-44b1-49bf-87a8-42b9e516f12f

crates/zed/src/main.rs

@@ -256,6 +256,7 @@ fn init_ui(
     project_panel::init(Assets, cx);
     outline_panel::init(Assets, cx);
     tasks_ui::init(cx);
+    snippets_ui::init(cx);
     channel::init(&app_state.client.clone(), app_state.user_store.clone(), cx);
     search::init(cx);
     vim::init(cx);