Opt-in std::simd support in zune-jpeg

[Shnatsel]

The std::simd module provides a safe, portable API for SIMD. Together with the multiversion crate it allows for SIMD that works from a single source file on every platform, and entirely in safe code!

The drawback is that it is nightly-only, and still technically unstable despite not having changed in quite some time. It would be great to add support for it and expose this as an opt-in feature, e.g. nightly-portable-simd. This would extend SIMD support to platforms not yet covered by intrinsics, such as WASM, and would allow people on nightly channel to use an implementation without unsafe code.

[etemesi254]

this would need porting of the idct function to simd, it's the only function that uses simd extensively

will see if feasible