feat(editor): flat block data (#9854)

Flat block data.

A new block type to flatten the block data

```typescript
// For developers
type Model = {
  blocks: Record<string, {
    flavour: string;
    cells: Record<string, {
      rowId: string;
      colId: string;
      text: Text;
    }>;
    cols: Record<string, {
      align: string;
    }>
    rows: Record<string, {
      backgroundColor: string;
    }>
  }>
}

// How it's saved in yjs
const yData = {
  blocks: {
    'blockId1': {
      flavour: 'affine:table',
      'prop:rows:row1:backgroundColor': 'white',
      'prop:cols:col1:align': 'left',
      'prop:cells:cell1:rowId': 'row1',
      'prop:cells:cell1:colId': 'col1',
      'prop:cells:cell1:text': YText,
      prop:children: []
    },
  }
}
```

blocksuite/framework/store/src/reactive/is-pure-object.ts

@@ -0,0 +1,8 @@
+export function isPureObject(value: unknown): value is object {
+  return (
+    value !== null &&
+    typeof value === 'object' &&
+    Object.prototype.toString.call(value) === '[object Object]' &&
+    [Object, undefined, null].some(x => x === value.constructor)
+  );
+}