Provide a way to return to the regular editor from the new Markdown experience

The new Markdown editing experience should provide a clear way to return to the regular Text Editor for the current document.

## Steps to reproduce
1. Open a Markdown document in the new Markdown editing experience.
2. Try to switch back to the regular editor.

## Expected
An obvious button, command, or menu action opens the current document in the regular Text Editor.
A third option in the toggle menu with a 'code' icon?

## Actual
There is no clear way to return to the regular editor from the new experience.

The existing experimental Markdown editor is tracked by #326540.**