chore: update changelog

[digama0]

Add an entry for #2247. I elected not to mention either of the two main features actually implemented (have _ and the hygieneInfo matcher) because they are of a more technical nature. The more user-visible effect is the unkeywordification of this.

[Kha]

My bad, presented like this it does sound like a straight bugfix, which we don't usually document. I was thinking about the changed scoping behavior of this, but it seems quite unlikely that anyone made use of it across macro scopes in a way that would now break. What do you think?

[Kha]

Or perhaps more succinctly: "this is now an identifier again that is implicitly introduced by anonymous have := with regular scoping. It used to be a keyword that was visible in all scopes and led to unexpected behavior when explicitly used as a binder name.

[digama0]

I was thinking about the changed scoping behavior of this, but it seems quite unlikely that anyone made use of it across macro scopes in a way that would now break. What do you think?

I don't think anyone has used this in a macro, at least not that I have seen in mathlib and certainly not in the two or more nested macro-macros case that would actually be affected by this.

I am fine with your rewording. I was probably too hard on the original version so it is probably best for you to phrase it.