Just return a boolean for the TreeWalker filter fn. This diverges from the spec,
but since the goal of this implementation is not to fully implement the spec
and we're never going to use a native implementation, this doesn't matter and
the code is easier to read when the function is just returning a boolean like
any normal filter function.
When cleaning up <br> elements, we need to accurately determine whether there's
text before and after it in the block to know whether it will introduce a
visible line break.
