Improve HTMLUtils.findBlocks performance

For #9717.
Unfortunately, I couldn't come up with a more general way of doing this in TokenUtils.

For testing, you can take https://gist.github.com/MarcelGerber/5c11f1031af292044708, placing your cursor in the script tags and pressing `Ctrl-Space`.
@MarcelGerber Thanks for digging into this one. That's an impressive improvement.

I'm wondering if the main slow down is in `TokenUtils.moveNextToken()` because it has to get the full line text for every token. I wonder if the line text can be cached in the "context" object? Brackets seems to choke of files with long lines in many places so that would provide a benefit for many other functions and not just this one. Care to give that a try?