LibJS: Define Utf16String::to_utf8 to convert Utf16String to String

Author: https://github.com/trflynn89
Commit: c79d20be58
Pull-request: https://github.com/SerenityOS/serenity/pull/17014
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibJS/Runtime/Utf16String.h

@@ -50,6 +50,7 @@ public:
     Utf16View substring_view(size_t code_unit_offset, size_t code_unit_length) const;
     Utf16View substring_view(size_t code_unit_offset) const;
 
+    ThrowCompletionOr<String> to_utf8(VM&) const;
     ThrowCompletionOr<DeprecatedString> to_deprecated_string(VM&) const;
     u16 code_unit_at(size_t index) const;