[TOC][new Beta Editor] TOC is not shown in new beta editor

### Is this a hackmd.io issue?

- [x] Yes, the bug happens on https://hackmd.io
- [ ] No. If it's a self-hosted CodiMD instance, please fire up the issue in the repo of CodiMD: https://github.com/hackmdio/codimd

### What's the problem

`[toc]` keyword is not parsed to correct HTML format in new preview editor.

#### Current behaviour

> Screenshot (optional)


Provide steps to reproduce:

1. Enable new beta editor preview
2. input `[TOC]` in editor
3. It shows `[TOC]` text only in new editor, while in old version editor, it shows table of content correctly.

If possible, create a note with reproducible content and share the link with us.

note link: https://hackmd.io/features#ToC

#### Expected behaviour


#### Environment

I use HackMD on:

**Desktop**
- OS: win11
- Browser: brave

I'm not sure if I should post any issue about new preview feature on Github here... or report it on Hackmd feedback system.

Non viene indicato alcun file sorgente o test. Abilita il nuovo editor beta, inserisci [TOC] e confronta il suo output con quello del vecchio editor; l'issue è risolta quando la parola chiave esegue il rendering di un indice invece del testo letterale.

Scritto dal modello di indicizzazione a partire dal testo della issue.