refactor(editor): adjust folder structure for slash menu extension (#10588)

Close [BS-2743](https://linear.app/affine-design/issue/BS-2743/slash-menu插件化：调整文件夹结构)

This PR move slash menu from `affine-root-block` to `widget-affine-slash-menu`, and make it as a `WidgetViewExtension`

blocksuite/blocks/package.json

@@ -43,6 +43,7 @@
     "@blocksuite/affine-widget-frame-title": "workspace:*",
     "@blocksuite/affine-widget-remote-selection": "workspace:*",
     "@blocksuite/affine-widget-scroll-anchoring": "workspace:*",
+    "@blocksuite/affine-widget-slash-menu": "workspace:*",
     "@blocksuite/affine-widget-toolbar": "workspace:*",
     "@blocksuite/block-std": "workspace:*",
     "@blocksuite/data-view": "workspace:*",