Explicitly set the contents of the npm package

This will reduce the size of the downloaded package by 25KB, which matters a lot for a package that is downloaded almost 40.000 times a day.

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> The "files" field is an array of files to include in your project. If you name a folder in the array, then it will also include the files inside that folder. (Unless they would be ignored by another rule.)

https://npmjs.org/doc/json.html