NC Text Editor: Add a raw text mode

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Is your feature request related to a problem? Please describe.

With the old text editor, it was either pure raw markdown or the rendered text. Now the WYSIWYG editor is a great improvement (thank you very much! 🤗) however a raw text mode is missing now.
Sometimes you want to edit certain markdown things, which are a bit awkward to do in the WYSIWYG. For example, you can not remove an inline code block without deleting the text.

Describe the solution you'd like
A button to switch the editor into a raw editor.

Describe alternatives you've considered
Every markdown you can use, should be revertible via the UI/keyboard shortcuts

Additional context
See also https://github.com/nextcloud/text/issues/3570

Start by locating Deck's text-editor integration and compare its behavior with the related nextcloud/text issue 3570. The work is done when users can switch to raw text editing and return to the editor while preserving the document's Markdown content.