Postinstall ngcc migration
Automatically adds a postinstall script to package.json to run ngcc.
If a postinstall script is already there and does not call ngcc, the call will be prepended.
Before
{
  "scripts": {
    "postinstall": "do-something"
  }
}
After
{
  "scripts": {
    "postinstall": "ngcc ... && do-something"
  }
}