💡 Add DefaultBlockData type to develop customized paragraph blocks

1. Describe a problem.
When splitting a paragraph block, Editor.js makes block data based on [editor-js/paragraph](https://github.com/editor-js/paragraph).

https://github.com/codex-team/editor.js/blob/next/src/components/modules/blockManager.ts#L488

This behavior may occur errors with customized default block.

2. Describe the solution you'd like. Mockups are welcome.
Add `DefaultBlockData` type in Editor.js.

```
interface DefaultBlockData {
text: string;
}
```

See also: https://github.com/hata6502/editorjs-examples/blob/main/examples/ExampleDefaultBlockTool.ts