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
Bennet Bo Fenner
63fe554c35