Group @testable imports at the bottom, per public/rules-swift's swiftformat. A
behavior-preserving import-ordering reformat across 252 files (mostly tests).
--trailingclosures was deliberately not adopted (cosmetic, and swiftformat flags
it as caution-worthy).
The fragment TreeBuilder gains the HTML5 active-formatting-list and reconstruct
step that the document builder already had: when a formatting element (b, i,
em, ...) is closed out of order, it is remembered and re-opened for the
following content. So HTML.parse now agrees with HTML.parseDocument and the
spec, e.g. <b><i></b>X</i> -> <b><i></i></b><i>X</i> and
<p>1<b>2<i>3</b>4</i>5</p> -> <p>1<b>2<i>3</i></b><i>4</i>5</p>. The conformance
corpus that surfaced this now checks the fragment path directly.
Closes#109
