Markdown widget parses backslash wrong in headers

**Describe the bug**
The Markdown widget fails to parse a backslash at the end of a first header line as a multi-line header. However it inserts the backslash it self with Shift + Enter

**To Reproduce**

1. Open a page with a markdown editor in rich text mode
2. Enter "First Second"
3. Insert line break with Shift + Enter after "First"
4. Select markdown mode
5. Observe a backslash is present in markdown
6. Select RichText mode
7. Observe that now a backslash is present the markup is not showing as a multi-line

**Expected behavior**
The Multi-Line header should be shown correctly.

**Screenshots**

**Applicable Versions:**

- netlify-cms-app 2.15.72
- netlify-cms-core 2.55.2
- netlify-cms 2.10.192
- Git provider: GitLab
- OS: Win 10
- Browser version v1.48.171 (Chromium 110.0.5481.177)

**CMS configuration**

```
Widget Config:
- { label: 'Textkörper', name: 'body', widget: 'markdown' }
```

**Additional context**
[Related issue](https://github.com/netlify/netlify-cms/issues/512) on backslash behavior in paragraphs