feat(storybook): Add 'build-storybook' command

[hscgavin]

To build the Storybook, first add the following npm script:

{
  "scripts": {
    "build-storybook": "sku build-storybook"
  }
}

Then run npm run build-storybook.

By default, Storybook assets are generated in the dist-storybook directory in your project root folder. If you would like to specify a custom target directory, you can provide it via the storybookTarget option in sku.config.js:

module.exports = {
  storybookTarget: './dist/storybook'
};

[markdalgleish]

Nice one—might even start using this on Braid 👏👏👏

[mengtzu]

These changes are very well commented, I understand the reasoning behind some otherwise weird looking code 👍