Fix warnings in enum.hpp high bit calculation

[jpeletier]

This PR fixes compilation warnings, at least in GCC, due to sign mismatch calculating high_bit in enum.hpp

After #696 was merged, a warning appeared when compiling enum.hpp:

/flecs/private/../addons/cpp/utils/enum.hpp:264:49: warning: overflow in conversion from ‘int’ to ‘flecs::_::enum_reflection<Ei8, flecs::_::enum_data_impl<Ei8>::reflection_count>::U’ {aka ‘signed char’} changes value from ‘128’ to ‘-128’ [-Woverflow]
  264 |     static const U high_bit = static_cast<U>(1) << (sizeof(U) * 8 - 1);

Inspite of the warning, the code works. The PR fixes this issue by calculating the high bit with an unsigned version, then casting back to the underlying enum type. This tells the compiler we really mean what we are doing.