Add script to generate npm-shrinkwrap.json

[web-padawan]

Connected to #515

Note: both the generated package.json and npm-shrinkwrap.json must be copied, committed and published during the release process, I haven't covered that part (need to configure CI).

So, to get this finished, we need following steps:

1. create a separate repo DONE: https://github.com/vaadin/vaadin-shrinkwrap

Release process config to be updated:

1. copy files to that repo
2. commit and push
3. run npm publish --access public in that repo

---

This change is 

[mehdi-vaadin]

Reviewed 4 of 4 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved