LibJS+LibUnicode: Port Intl.DateTimeFormat to UTF-16 strings

Author: https://github.com/trflynn89
Commit: ee01f857d1
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5581

Libraries/LibUnicode/ICU.cpp

@@ -162,6 +162,11 @@ String icu_string_to_string(UChar const* string, i32 length)
     return MUST(Utf16View { string, static_cast<size_t>(length) }.to_utf8());
 }
 
+Utf16String icu_string_to_utf16_string(icu::UnicodeString const& string)
+{
+    return Utf16String::from_utf16_without_validation({ string.getBuffer(), static_cast<size_t>(string.length()) });
+}
+
 UCharIterator icu_string_iterator(Utf16View const& string)
 {
     UCharIterator iterator;