WASM has ctz and it's honestly the better place for such a low level instruction. BigInt is getting some of the bitwise ops (https://github.com/tc39/proposal-bigint/blob/master/ADVANCED...) but just the higher level ones at this time.

JavaScript does have clz32 from back before WASM was the new target for compile-system-language-to-browser but not ctz32 and I doubt it'd be added as that's no longer the focus.