`syz` function with `slimgb` option

[zengmao]

In Singular, the syz function for computing syzygies can accept an optional argument specifying the Groebner basis algorithm, which can be either "std" or "slimgb":
https://www.singular.uni-kl.de/Manual/4-4/sing_369.htm#SEC409

Currently, Singular.jl only exposes the syz function with one argument, the ideal. Allowing a second argument would allow significant performance improvements in some situations.