Update .npmignore

Per https://github.com/paulmillr/Array.prototype.findIndex/commit/11e9fce59c758dddd2810e60de91223d9a78bb62#commitcomment-8901378, when a `.npmignore` is absent, `.gitignore` is used - now that `.npmignore` is present, these other things should continue to be npm ignored.
Merge pull request #12 from ljharb/patch-2

Update .npmignore