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/src/effects.ts

@@ -46,6 +46,7 @@ import { effects as widgetEdgelessAutoConnectEffects } from '@blocksuite/affine-
 import { effects as widgetFrameTitleEffects } from '@blocksuite/affine-widget-frame-title/effects';
 import { effects as widgetRemoteSelectionEffects } from '@blocksuite/affine-widget-remote-selection/effects';
 import { effects as widgetScrollAnchoringEffects } from '@blocksuite/affine-widget-scroll-anchoring/effects';
+import { effects as widgetSlashMenuEffects } from '@blocksuite/affine-widget-slash-menu/effects';
 import { effects as widgetToolbarEffects } from '@blocksuite/affine-widget-toolbar/effects';
 import { effects as stdEffects } from '@blocksuite/block-std/effects';
 import { effects as dataViewEffects } from '@blocksuite/data-view/effects';
@@ -103,6 +104,7 @@ export function effects() {
   widgetRemoteSelectionEffects();
   widgetDragHandleEffects();
   widgetEdgelessAutoConnectEffects();
+  widgetSlashMenuEffects();
   widgetToolbarEffects();
 
   fragmentFramePanelEffects();

blocksuite/blocks/src/index.ts

@@ -114,6 +114,9 @@ export {
 export type { DragBlockPayload } from '@blocksuite/affine-widget-drag-handle';
 export { toolbarWidget } from '@blocksuite/affine-widget-toolbar';
 
+// TODO(@L-Sun): Narrow the export scope
+export * from '@blocksuite/affine-widget-slash-menu';
+
 const env: Record<string, unknown> =
   typeof globalThis !== 'undefined'
     ? globalThis