NextJS: Allow disabling next/image lazy loading

[martinnabhan]

What I did

next/image lazy loads images by default. When using Storybook (and especially Chromatic) we usually want images below the fold to load as well so they show up in snapshots. This pull request sets next/image's loading property to eager which disables lazy loading.

How to test

1. Create a story using next/image and place an image below the fold
2. Open Storybook in your browser
3. Access the story and make sure the image gets loaded even if it's below the fold (or use Chromatic to snapshot the story and make sure images below the fold are loaded and visible)

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]

[valentinpalkovic]

@martinnabhan Thank you so much for your contribution.

Your PR would introduce a breaking change where people might rely on lazy loading behavior. Instead, this option should be configurable. Let's add it to the parameters.nextjs object instead, where people like you could define the default loading behaviour.

[martinnabhan]

I thought that might be the way to go! I'll see if I can turn it into a parameter some time this week.

[martinnabhan]

@valentinpalkovic Thank you for approving! What's the status on this? build-sandboxes seems to be failing, but it doesn't seem to be related to this pull request.