Fix #2516, #3560: Unicode space handling

[lydell]

It is possible to match only valid JavaScript identifiers with a really long
regex (like coco and CoffeeScriptRedux does), but CoffeeScript uses a much
simpler one, which allows a bit too much.

Quoting #1718 #issuecomment-2152464 @jashkenas:

But it still seems very much across the "worth it" line. You'll get the
SyntaxError as soon as it hits JS, and performance aside -- even the increase
in filesize for our browser coffee-script.js lib seems too much, considering
this is something no one ever does, apart from experimentation.

In short, CoffeeScript treats any non-ASCII character as part of an identifier.
However, unicode spaces should be excluded since having blank characters as part
of a word is very confusing. This commit does so, while still keeping the
regex really simple.

[jashkenas]

Sounds alright. I guess you're imagining some really bizarre copy-and-paste scenarios?

[lydell]

Yes. Most importantly the “I accidentally pressed AltGr+Space which inserted a non-breaking space” scenario, though.

[michaelficarra]

👍

[loveencounterflow]

+1. BTW it also happened to me when testing out some Chinese character processing stuff and not switching the keyboard back to Western before editing outside a string literal; you then get U+3000 ideographic spaces with the spacebar; i'd say there quite a few whitespace characters out there you don't want to see except in a string (i'm +1 for outruling tabs as indentation, too).