Use mime-score to resolve extension conflicts.

[broofa]

[See #42 for long-ago discussion of this idea.]

As noted in that old PR, I created the mime-score module to encapsulate the logic used by mime to resolve extension conflicts in mime types. I think it would be helpful to users of mime and mime-types if both libs used the same logic.

I'll put up a PR for that (again), but the main issue with that previous PR was that mime-score didn't support node@0.6. I suspect that's no longer a concern? It'd be nice to not have to worry about that, and to be able to develop using reasonably modern language features. See #115 for further discussion there.

[Edit to add: The current support matrix is maybe even more relevent than I thought. 'Just realized that I ported mime-score to typescript last year, and it's currently built for ESNext. So, it works on node@14 but breaks on node@12.]