Convert `minify-trace` to library code

We'd like to make use of `lighthouse-core/scripts/lantern/minify-trace.js`, but at this moment it's a script to be run on the command line and outputs a file to disk (it has no exports).

Would it be possible to move it into `lighthouse-score/lib/lantern`, adding an export then updating the existing `lighthouse-core/scripts/lantern/minify-trace.js` to use the lib code?