removeScriptAndStyle re-searched <script, <style and <!-- from the cursor
on every iteration and, for an unclosed opener, scanned to the end for
its closing tag once per occurrence; convertBlockTagsToNewlines rescanned
to the end for '>' at every '<'. An ingested ~1 MB file of unclosed
'<script' openers, empty comments or bare '<' took over a minute.
Cache each next-occurrence position until the cursor passes it and
remember when a closing token is absent from the rest of the document.
The same three inputs now finish in well under a second.
