ISUPPORT value escaping/unescaping

[DanielOaks]

I’ve always sorta ignored it as unsupported but if servers are sending values escaped that way (esp for spaces etc) it makes sense to have consistent implementations of it and all.

note to self: don’t mandate converting all utf-8 chars to the escaped forms, I could def see servers just using the encoding opportunistically for mostly stuff like e.g. space that’s not possible to represent b/c protocol limitations and junk, e.g. https://github.com/inspircd/inspircd/blob/6cfabb0/src/server.cpp#L176-L182