feat: render placeholder in edgeless mode (#11387)

Complete [BS-2997](https://linear.app/affine-design/issue/BS-2997/在白板上渲染-inserted-frame-group-时提供占位)

blocksuite/framework/std/src/gfx/index.ts

@@ -52,7 +52,7 @@ export {
   GfxCompatibleBlockModel as GfxCompatible,
   type GfxCompatibleProps,
 } from './model/gfx-block-model.js';
-export { type GfxModel } from './model/model.js';
+export { type GfxModel, isPrimitiveModel } from './model/model.js';
 export {
   convert,
   convertProps,

blocksuite/framework/std/src/gfx/model/model.ts

@@ -1,7 +1,7 @@
 import type { GfxGroupCompatibleInterface } from './base.js';
 import type { GfxBlockElementModel } from './gfx-block-model.js';
-import type {
-  GfxGroupLikeElementModel,
+import {
+  type GfxGroupLikeElementModel,
   GfxPrimitiveElementModel,
 } from './surface/element-model.js';
 
@@ -10,3 +10,9 @@ export type GfxModel = GfxBlockElementModel | GfxPrimitiveElementModel;
 export type GfxGroupModel =
   | (GfxGroupCompatibleInterface & GfxBlockElementModel)
   | GfxGroupLikeElementModel;
+
+export const isPrimitiveModel = (
+  model: GfxModel
+): model is GfxPrimitiveElementModel => {
+  return model instanceof GfxPrimitiveElementModel;
+};