Implementing C23 _Float16 in ImportC

[Richard (Rikki) Andrew Cattermole]

I forgot to introduce this for a decision at the last meeting (lucky 
given how many items we had).

A new policy, that anyone could implement a C numeric type as long as it 
bound to a druntime module, and not exposed to D. D would need a DIP due 
to the identifier needing confirmation.

Note: this would also mean anyone could implement (u)cent and it would 
be pre-approved work.

Also did you see my email from the day after the meeting?