[lib] Make skim::prelude::reader and skim::prelude::engine::fuzzy public

[PanAeon]

Is it possible to make skim::prelude::reader and skim::prelude::engine::fuzzy public? Right now there's an option cmd_collector, but since CommandCollector is private it's impossible to pass custom value to it. (or it may be just me).
Also when you can provide your own command collector, there's no point in reimplementing FuzzyEngine. I think making FuzzyEngine public will only benefit the library