Support Preferences.jl to set default executor

[schlichtanders]

Users may like to use Dagger.jl as the default executor because it parallelizes to both Threads and Processes at the same time which is most intuitive.

Preferences.jl is perfect for this, as it allows for static parameters which trigger a precompilation on change. So there is no performance penalty.

It would be great to use Transducers with a custom default.