When inserting a line break in a contenteditable with preformatted
white-space (pre, pre-line, pre-wrap), insert a newline character (\n)
instead of a <br> element. Use <br> only for padding at end of line to
ensure the cursor can be placed on the new line.
The editing command that relies the most on this, `insertLinebreak`,
did not perform a layout update after inserting a `<br>` which caused
this algorithm to always return false. But instead of actually building
the layout tree needlessly, we can check the DOM tree instead.
