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 on backslash behavior in paragraphs