improve extractor to better match 'refmt:ed' pxx syntax

[J-Zeitler]

This change enables support for two cases of "refmt":ed pxx syntax as reported in #115

Note: extractor now does not consider the []-brackets normally surrounding the %tw ppx macros

[dylanirlbeck]

Overall, the changes LGTM (and thanks for the nicely-written tests!)

I tried to imagine a situation where the regex might incorrectly match something after removing the [ requirement, but I couldn't think of anything. I'll go ahead and merge this in!

[dylanirlbeck]

@all-contributors Please add @J-Zeitler for code

[allcontributors]

@dylanirlbeck

I've put up a pull request to add @J-Zeitler! 🎉

[J-Zeitler]

Cool, I was thinking about how this might break stuff. The important part should be to match %tw as a unique identifier for "tailwind-ppx syntax". If refmt wants to get rid of the hardbrackets, then I guess ppx:es cannot really count on them being part of the ppx-syntax.

(props for the speedy review by the way :) let me know if things start to behave incorrectly)