`x4::char_set` must be capable of propagating the constexpr-initialized variable to runtime

[saki7]

Currently, x4::char_set internally uses x4::detail::basic_chset which holds the internal buffer by x4::detail::char_range_run that uses std::vector.

In theory, this internal buffer can be totally constexpr if we do the parser construction in two phases and statically determine the maximum buffer amount in consteval preparation step (note: NOT constexpr). We can use std::inplace_vector (C++26; polyfill needed in iris-cpp/iris) for implementation details.