Previously it used `devDep` of `package.json`. It was working but doing
so all Storybook packages had to be added to `devDep`.
Moreover, as `@storybook/react-native` was not part of the monorepo
anymore it's version was stuck to `6.0.0-alpha.0` causing a download
of `6.0.0-alpha.0` SB packages when running `yarn install`.
It move the `spawnSync` usage to only a single place which is simpler to maintain.
It also simplify testing as there is no more need to mock an external dep
The goal of this will be to use the Factory Pattern to encapsulate everything specific to a package manager in a single class.
Then a factory will return an instance of NPM, Yarn or Yarn 2 proxy class according to the package manager used/prefered in end project.
All CLI code will be package manager agnostic and will just call functions of JsPackageManager interface.
