This could be accomplished with just a css snippet like:
div.markdown-source-view {
color: transparent;
}
That makes the text in edit mode transparent so it’s invisible. To see what you have typed you could just toggle Preview Mode.
You could also create a snippet to make the text invisible in both edit and preview mode and use a plugin like Snippet Commands or MySnippets to quickly toggle the snippet on and off.
Note: I’m not an insider so dunno if some of this won’t work