Add yet another tree walking utility: CommentUtils::linearWalk().
Unlike TreeWalker, it allows handling the beginnings and ends of nodes
separately – kind of like parsing a XML token stream, or kind of like
VisualEditor's linear model.
(Add unit tests for this utility. The simple.html test case is copied
from [VisualEditor/VisualEditor]/demos/ve/pages/simple.html.)
Use this utility to stop skipping when we reach either a closing or
opening block node tag. Previously we'd skip over such tags inside
nested "transparent" nodes (like <a>, <del>, or apparently <font>).
Bug: T271385
Change-Id: I201a942eb3a56335e84d94e150ec2c33f8b4f4e0
