While editing, we need to consider whether removing a <br> has any
effect on layout to determine whether its extraneous. This new condition
finds most cases for extraneous <br>s inside block elements.
We currently delete a single code unit. If the user presses backspace on
a multi code point emoji, they are going to expect the entire emoji to
be removed. This now matches the behavior of Chrome and Firefox.
The spec calls for a couple of very specific whitespace padding
techniques whenever we canonicalize whitespace during the execution of
editing commands, but it seems that other browsers have a simpler
strategy - let's adopt theirs!
