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/affine/shared/src/utils/index.ts

@@ -14,6 +14,7 @@ export * from './insert';
 export * from './is-abort-error';
 export * from './math';
 export * from './model';
+export * from './popper-position';
 export * from './print-to-pdf';
 export * from './reference';
 export * from './reordering';