Support fine grained sideEffects field in package.json

[devongovett]

The current implementation of tree shaking supports sideEffects: false, but some libraries need more fine grained control to allow a subset of files to include side effects. webpack allows this by supporting an array of files which contain side effects.

{
  "sideEffect": ["./foo.js", "./bar.js"]
}

For compatibility with libraries that depend on this, we should support it in Parcel as well.

[ahocevar]

I've implemented support for the fine grained sideEffects syntax and submitted it as #1766.