Add QString::toStdU8String and QString::fromStdU8String

Qt already has functions to convert from and to std::u16string / std::u32string.
Since c++20 and c23, we have char8_t. It would be good, to also have functions which convert from/to std::u8string.

QString::toStdU8String is missing and should look like this for pre c++20:

inline std::string QString::toStdU8String () const
{ return toUtf8().toStdString(); }

for c++20:
QString::toStdU8String is missing and should look like this:

inline std::string QString::toStdU8String () const
{ return toUtf8().toStdU8String(); }

Optional:
I also noticed that the `QString::toStdString` function is ambiguous: C++ does not specify an encoding for it, but it is only used together with the locale encoding. This makes the function unusable in cases where the `std::locale` is not based on UTF-8. This is particularly fatal in China, where UTF-8 is practically forbidden as the default on desktops (even on UNIX Systems). But also on Windows computers world wide, which typically do not use utf-8 as default, instead ANSI is used.

After the discussion below, I won't propose anymore to deprecate it, but rather to also add `QString::from/toLocalStdString`.