`X4Movable` (all overloads of `x4::move_to(src, dst)`) should be SFINAE-friendly

[saki7]

Some classes use X4Movable to show human-friendly errors but in return the current implementation of X4Movable is not SFINAE-friendly in certain scenario.

Generically speaking, any concept should be SFINAE-friendly. We should fix X4Movable while keeping human-friendly errors in all usages.

The overload below should ideally be = delete("with reason"); but it's not implemented in C++23. How can we solve this issue?

x4/include/iris/x4/core/move_to.hpp

Lines 68 to 84 in e811b03

	template<traits::NonUnusedAttr T>
	constexpr void move_to(T&, T&) noexcept
	{
	static_assert(
	!std::is_const_v<T>,
	"`x4::move_to(T const&, T const&)` is not allowed"
	);
	static_assert(
	false,
	"lvalue reference detected on the `src` argument of `x4::move_to`. "
	"The caller is definitely lacking `std::move` or `std::forward`. If you "
	"intend to *copy* the mutable value, apply `x4::move_to(std::as_const(attr_), attr)`."
	);
	// Banned: possible, but bug-prone.
	// dest = std::move(src);
	}