milkdown: [Bug] Multiline headings, GFM-plugin, switches to CommonMark style

Initial checklist

• I agree to follow the code of conduct
• I searched issues and discussions and couldn’t find anything (or linked relevant results below)

Affected packages and versions

@milkdown/preset-gfm

Link to runnable example

No response

Steps to reproduce

Create a Milkdown setup, with an “H1” element, use shift+enter

Expected behavior

Should add, to be aligned with other modules, add a \ newline, not switch to “non-gfm” header style.

# Milkdown 💗 React

what```

### Actual behavior

```# Milkdown 💗 React```

```Milkdown 💗 Reactin\
what
====```

### Runtime

_No response_

### OS

_No response_

### Build and bundle tools

_No response_