printf %Hf, %Df, etc. for _Decimal types

[Steve Summit]

Now that both C23 and gcc support _Decimal64 and the other
decimal floating point types, I'm thinking of tackling the
problem of adding proper support for %Df and the like to glibc's
printf.

I'm sure this has been thought about before; I don't know if
there's a reason it hasn't been completed yet.  (I do know about
add-ins like libdfp, but what I'm thinking about now is full,
integrated, out-of-the-box support.)

If anyone has any advice or suggestions, I'm open to hearing
them.  If there are good reasons this hasn't been done, I'm
interested; if someone has already thought about the "right"
way to implement it, I'm interested.  (I'm no expert on glibc,
so left to my own devices I might well choose a suboptimal
approach.)  If someone is already working on this, obviously
I'd like to know that, too.  Finally, if there's a better mailing
list (libc-alpha?) to reach someone who might have answers to
these questions, that would be good to know, too.

Steve Summit