We certainly don't recommend giving users access to UNA Studio... :)

The Froala editor can probably be edited to do what you are looking for. You can edit the buttons available in Studio>Settings>Froala.  Go to https://www.froala.com/wysiwyg-editor/examples/toolbar-buttons as take a look at what is possible there, you can add a button for CodeView, perhaps if that is all you want, you could remove all the WYSYWIG functions?
Perhaps using Posts module?