Add support for JS backend

[Yu-Vitaqua-fer-Chronos]

Hey there, I've made a library (NULID) and I wanted to implement support for the JS backend, but it seems like nint128 doesn't support the JS backend, but it does support the NimVM, so I'm wondering if it'd be possible to also support JS at the same time?

[rockcavera]

The problem with the JS backend is that it does not support 64-bit integers and the Int128 and UInt128 object use 64-bit integers.

At the time I designed nint128 I did not intend to support the JS backend, as there are libraries for large integers in JS.

See also this answer given on the forum.

[Yu-Vitaqua-fer-Chronos]

Thank you for your response! I'll likely use BigInts then in my library for this use, and thank you for this amazing library :)