Turns out Editor::newline_below was not respecting the read_only flag because it uses buffer.edit. Editor::newline and Editor::newline_above behaved correctly because they used editor.edit which ignores edits if read_only is true.

Added an early return if the editor is read only, also added it to newline/newline_above for clarity.

Release Notes:

• Fixed an issue where newlines could be inserted into read-only editors