Fix: dedupe addons when merging storybook main config

[jtoar]

Since we added @storybook/addon-essentials #4765, users that already have it in their storybook configs get the following error:

ERR! ValidationError: Invalid configuration object. Webpack has been initialized using a configuration object that does not match the API schema.

The reason is that webpack-merge doesn't seem to dedupe keys that have arrays. It just seems to concat them together. So if a user's storybook config in web/config/storybook.config.js is...

module.exports = {
  /**
   * This line adds all of Storybook's essential addons.
   *
   * @see {@link https://storybook.js.org/addons/tag/essentials}
   */
  addons: ['@storybook/addon-essentials'],
}

The result of this merge here:

redwood/packages/testing/config/storybook/main.js

Line 157 in 2145f44

return merge(baseConfig, userStorybookConfig)

is...

{
  addons: [
    '@storybook/addon-essentials',
    '@storybook/addon-a11y',
    '@storybook/addon-essentials'
  ],
  // other keys...
}

So this PR dedupes that key.

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[jtoar]

@virtuoushub thoughts on if it makes sense to add a test for this?

[virtuoushub]

@virtuoushub thoughts on if it makes sense to add a test for this?

@jtoar my vote is that I'd rather get this fix out; I will add a line item to add tests for stuff like this in #5269

[virtuoushub]

Great find, thanks for this fix!