Add lexer for the Clean language

[camilstaps]

Some notes:

• Clean is a functional programming language similar to Haskell. Because Clean uses different comment markers, Haskell lexers will immediately fail on almost any Clean code.
• Some regexes in here were stolen from https://gitlab.science.ru.nl/cloogle/clean-highlighter/blob/master/clean.js which I maintain. It attempts to recognise many more fine distinctions but is because of that unfortunately more buggy. In here I tried to be more minimal.
• The code { ... } construct allows one to use inline ABC code (comparable to inline assembly code in C).

Any suggestions would be welcome! Thanks for taking the time to review 🙇‍♂️

[pyrmont]

@camilstaps Thanks for the PR! I'll try to look at this over the next couple of days :)

[camilstaps]

That's great, @pyrmont! You have done an amazing job with catching up with the backlog so far, as far as I can see 👍

[pyrmont]

Please let me know if any of the comments are unclear! (Or if I've made a mistake!)

[camilstaps]

Thanks @pyrmont. I believe I addressed all your questions and added examples (for the latter I did not reply to your comments). The sample has also been shortened significantly. I'm only unsure what to do with the . of ABC keywords.

[camilstaps]

To be clear, Clean's ABC language is different from the musical notation language for which a lexer was proposed in #1092.

[pyrmont]

@camilstaps Thanks for replying so quickly! Will try to get back to you with similar speed! :)

[pyrmont]

@camilstaps I think there are only two comments outstanding at this stage. Let me know if anything is unclear!

[pyrmont]

Thanks for your submission, @camilstaps! Always good to have another language be part of Rouge :)

We're currently following a two-week cadence for minor releases and this code is scheduled to land in v3.10.0 on Tuesday 3 September 🎉