Since D1628R0 (Unicode character properties) conflicts with my proposal,
I decided to finish a draft and publish it:

https://github.com/Lyberta/cpp-unicode-fundamental

It proposes 5 strong types which are intended as the basis for the rest
of the Unicode library:

std::unicode::utf8_code_unit
std::unicode::utf16_code_unit
std::unicode::utf32_code_unit
std::unicode::code_point
std::unicode::scalar_value