fix: deprecated use of std::char_traits<unsigned short>::length

[Clement-Devevey]

Fix the warning about deprecated use of char_traits by char_traits<char16_t> as it has been discussing in this thread:
@codeinred Since we've decided to make C++11 a requirement for future versions of JavaCPP, let's just replace unsigned short with char16_t. Thanks!

Originally posted by @saudet in #753 (comment)

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Original issue is;
warning: 'char_traits' is deprecated: char_traits for T not equal to char, wchar_t, char8_t, char16_t or char32_t is non-standard and is provided for a temporary period. It will be removed in LLVM 19, so please migrate off of it. [-Wdeprecated-declarations]

[Clement-Devevey]

Having one issue about doing this:
in JavaCPP_createStringFromUTF16 the return env->NewString(ptr, length); isn't working:
Cannot initialize a parameter of type 'const jchar *' (aka 'const unsigned short *') with an lvalue of type 'const char16_t *'

[Clement-Devevey]

Apparently this is safe to cast from char16_t to jchar

It is generally safe to cast char16_t* to jchar* in JNI, as both types are 16-bit wide and represent UTF-16 encoded characters. This approach is commonly used in JNI code to handle UTF-16 strings.

Here’s a brief explanation:

char16_t: A 16-bit character type introduced in C++11 to represent UTF-16 encoded characters.
jchar: A 16-bit type used by JNI to represent UTF-16 encoded characters, which is typically defined as unsigned short.
Since both char16_t and jchar are 16-bit wide and represent UTF-16 characters, casting between them is safe and does not alter the data. This practice is used in various JNI implementations.µ

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Using reinterpret_cast ensures that the conversion is done correctly at the bit level, which is necessary for this specific case in JNI where char16_t and jchar are essentially the same type but with different names.

[saudet]

There a few other places where we could switch from unsigned short to char16_t, so we might as well do it now?

[saudet]

It looks like it doesn't work at all if we don't change all unsigned short to char16_t, so please do that! Thanks