deprecate multiple-underscore-only variable names

[stevengj]

We already deprecate _ as a variable name. Hopefully, we can eventually use it as a placeholder for discarded lhs values (#9343) and/or for a compact anonymous-function/currying syntax (e.g. _.a for x -> x.a: #554, #5571, #22710).

As I discussed with @Keno today, it might be good to additionally deprecate all variable names consisting only of underscores, e.g. __ or ___. It is weird to disallow _ but allow __, and multiple underscores might be useful for something later.

(One possible use: in an implicit currying syntax, make f(g(_,y)) denote f(x -> g(x,y)) and f(g(__,x) denote x -> f(g(x,y)), i.e. _ is "tight" and __ is "loose".)

[StefanKarpinski]

+1 to deprecation, whether we want this to mean anything is a different matter.

[StefanKarpinski]

Just to be conservative, we should deprecate these. If they're deprecated we can always change our minds but we can't go in the other direction.