* feat: __testEditable allows editable region tests
It's not entirely isolated, but it makes it possible to select only the
element with id 'editable-only' which is built solely from code inside
the editable region.
* fix(client): missing editableContents -> ''
Previously it was added as the string 'undefined'
* fix: more informative error messages
* fix: DRY, correct and test getLines