Default to using Dagger.jl as the backend

[schlichtanders]

As a user of Folds.jl I would like to have things just work as good as possible from the start. Currently Folds.jl runs on Threads, however if I have multiple machines available, I as a user of course would like parallelize upon all Threads on all machines.

Dagger.jl does indeed use both Threads and Machines by default. Hence making the Dagger Backend the default backend looks like it makes sense for Folds.jl