[BUG] insertMarkdown() doesn't work correctly when editing in "source" mode

[zjullion]

• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• I have read the documentation and cannot find an answer.

Describe the bug
insertMarkdown() doesn't work correctly when editing in "source" mode with the diffSourcePlugin - specifically, any edits made in "source" mode will be undone, and the text will be inserted based on the last state from "rich-text" mode.

Reproduction
See https://codesandbox.io/p/sandbox/mdx-editor-base-forked-tzrt6v

To Reproduce

1. Click the toggle into source mode
2. Add some markdown
3. Click the "Insert Markdown" button
4. See that your changes are lost

Expected behavior
I would expect insertMarkdown() to behave the same way in "rich-text" and "source" mode.

Screenshots
N/A

Desktop (please complete the following information):

• OS: Windows 11
• Browser: Chrome

Additional context
N/A

[petyosi]

@zjullion this is not implemented indeed. Happy to accept a PR.