extendPackage changes the order of the keys

[elevatebart]

Version

3.4.1

Reproduction link

https://github.com/elevatebart/test-vue-lib

Environment info

MacOS, node 10, yarn 1.13

Steps to reproduce

When using the followin code in my plugin, I get changes on the order of keys in packages.json. It is very unsetling for users.

api.extendPackage({
    scripts: {
      styleguide: 'vue-cli-service styleguidist',
      'styleguide:build': 'vue-cli-service styleguidist:build'
    }
  })

What is expected?

I expect the extendPackages touches only what it is intended to and leves the rest alone.

What is actually happening?

The folloing keys are moved to the end of the file:
main, files, unpkg

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Found while inverstigating vue-styleguidist/vue-cli-plugin-styleguidist#34

[elevatebart]

peerDependencies is faulty here as well.