Circular imports

[millette]

First of, thanks for the package. I've not had a change to try it but it's on my short list.

I was going over the code when I noticed engine.ts and bktree.ts import each other.

Would you accept a PR extracting damerauLevenshteinDistance and DistanceCache to a separate file? At first glance, I think that would break (fix) the cycle.