Make backends accept a function that takes the available backends as an argument

[jiribenes]

Avoid having to reference platform-specific libraries by changing the backends argument everywhere to

  backends ? (bs: with bs; [ js ])  # Backends is now a function taking available backends

and then on a use site change to something like:

    buildInputs = [pkgs.jre] ++ pkgs.lib.concatMap (b: b.buildInputs) (backends effektBackends);

Don't forget about the assert that the resulting list of backends shouldn't be empty, this probably needs a let binding.
It might be also useful to check that backends is a function and report that to the user, especially given that it's a breaking change :)

Maybe the existing effektBackends needs to take pkgs as a parameter?