Damerau-Levenshtein distance

A direct port of the Levenshtein/Damerau-Levenshtein algorithm from natural to an N-API/C++ extension.

In the browser, it will use the implementation copied from natural, with the addition of an offset parameter in search results.

In NodeJS, it uses the N-API implementation. In tests on production data on our workload, it is ~15x faster than the original JS; on smaller strings as used in the automated tests, the speedup is ~27x.

Correctness is based on the assumption that natural is correct. :-) Compliance is currently verified by running distance checks on several thousand random sample strings and verifying that the N-API generates identical results.