Unclosed script/style tag handling Fixes #1614

[gerner]

Explicitly handle unclosed <script> and <style> tags which previously
would result in O(n^2) work to lex as Error tokens per character up to
the end of the line or end of file (whichever comes first).

Now we try lexing the rest of the line as Javascript/CSS if there's no
closing script/style tag. We recover on the next line in the root state
if there is a newline, otherwise just keep parsing as Javascript/CSS.

This is similar to how the error handling in lexer.py works except we
get Javascript or CSS tokens instead of Error tokens. And we get to the
end of the line much faster since we don't apply an O(n) regex for every
character in the line.

I added a new test suite for html lexer (there wasn't one except for
coverage in test_examplefiles.py) including a trivial happy-path case
and several cases around <script> and <style> fragments, including
regression coverage that fails on the old logic.

[gerner]

Also, I ran a real-world webpage (https://www.atlassian.com/git/tutorials/rewriting-history) through pygmentize with html formatting on both the old code and the new and got the same pygmentized output. So happy path seems to be unaffected.