stringToUTF16 trows "Cannot mix BigInt and other types" for wasm64

$em++ --version
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 4.0.15 (09f52557f0d48b65b8c724853ed8f4e8bf80e669)

The stringToUTF16 implementation (and most likely UTF32 and other variants) mixes BigInt and Number when doing math on the outPtr.

Can be reproduced with this example:

[main.cpp](https://github.com/user-attachments/files/22423273/main.cpp)