perf(Stats): use Web Workers

[maxpatiiuk]

Stats generation, when run on my text-hoarder-store, takes 7 min with --precise-stats and 40s without --precise-stats
By using web workers, we can make this a lot faster, though that would add complexity, and make code reuse between stats and pre-processor harder.
Plus, I am not running stats generation often, so performance is not important.
And --precise-stats aren't too much more precise than the default, at a large performance penalty

Thus, it might be best to consider using web workers only in the future, once pre-processor code is finished, and once there are more users of the Cli