I wanted to tinker with Private Use Area code points and I thought what
would be the standard way to do that. And it is obvious that we need a
UCD class and use it to hold the user-defined properties of Private Use
Area code points.

This is completely opposite of P1628.

We define a new class std::unicode::character_database (established
name, although I'd prefer calling it std::unicode::code_point_database)
and similarly to std::source_location it has static function default()
that will return the reference to const default UCD. Obviously, the
entire class should be constexpr.

Now, my initial idea was to make only Private Use Area mutable while the
rest is const but then it occurred to me that it will deny some nice
possibilities such as 3rd party libraries shipping older and newer
versions of UCD that is shipped with the standard library.

Essentially, I think the entire database should be mutable.
std::unicode::character_database should be a value type but I guess we
may word it so it is possible to implement mutations from default as a
separate internal state while the rest of the database simply links to
default one, like copy-on-write.

I don't have the API or the wording yet. This is just thoughts at this
point but it seems like P1628 is not the way to go at all.