use crossplatform parallel backend in code.R

[maxheld83]

Some participants on windows machines seemed to be confused by the {doMC} based parallel computing in code.R.
Might be nice to have a crossplatform backend in code.R to avoid that source of confusion, though that would depend on tidymodels/tune#157.
Don't know enough about the internals of tune or parallel on windows, so I can't solve this myself 😞.