Closes #21579

What I did

Updated the StorybookConfig interface to include the previewMainTemplate option because otherwise it would give a typescript error if users tried to include this valid config option.

How to test

1. Modify main.ts in ui/.storybook to include the previewMainTemplate option. The default one here could work as an example value for this option.
2. Adding this option should not give a typescript error.
3. Start the storybook dev server
4. All stories should load properly within the new preview template.

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"]