Add a reduce-motion CSS style for PreviewBlock.

[tofumatt]

Summary

Addresses issue:

• Enhance PreviewBlock for compatibility with prefers-reduced-motion #5055.

PR Author Checklist

• My code is tested and passes existing unit tests.
• My code has an appropriate set of unit tests which all pass.
• My code is backward-compatible with WordPress 4.7 and PHP 5.6.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.
• I have added a QA Brief on the issue linked above.
• I have signed the Contributor License Agreement (see https://cla.developers.google.com/).

---

Do not alter or remove anything below. The following sections will be managed by moderators only.

Code Reviewer Checklist

• Run the code.
• Ensure the acceptance criteria are satisfied.
• Reassess the implementation with the IB.
• Ensure no unrelated changes are included.
• Ensure CI checks pass.
• Check Storybook where applicable.
• Ensure there is a QA Brief.

Merge Reviewer Checklist

• Ensure the PR has the correct target branch.
• Double-check that the PR is okay to be merged.
• Ensure the corresponding issue has a ZenHub release assigned.
• Add a changelog message to the issue.

[github-actions]

Size Change: +152 B (0%)

Total Size: 1.36 MB

Filename	Size	Change
./dist/assets/css/googlesitekit-admin-css-********************.min.css	45.5 kB	+49 B (0%)
./dist/assets/css/googlesitekit-adminbar-css-********************.min.css	10.9 kB	+58 B (+1%)
./dist/assets/css/googlesitekit-wp-dashboard-css-********************.min.css	6.33 kB	+45 B (+1%)

ℹ️ View Unchanged

Filename	Size
./dist/assets/js/31-********************.js	51.9 kB
./dist/assets/js/32-********************.js	3.12 kB
./dist/assets/js/analytics-advanced-tracking-********************.js	769 B
./dist/assets/js/googlesitekit-activation-********************.js	33.2 kB
./dist/assets/js/googlesitekit-adminbar-********************.js	36.8 kB
./dist/assets/js/googlesitekit-api-********************.js	9.22 kB
./dist/assets/js/googlesitekit-base-********************.js	1.13 kB
./dist/assets/js/googlesitekit-dashboard-********************.js	61.6 kB
./dist/assets/js/googlesitekit-dashboard-details-********************.js	60 kB
./dist/assets/js/googlesitekit-dashboard-splash-********************.js	74.7 kB
./dist/assets/js/googlesitekit-data-********************.js	2.09 kB
./dist/assets/js/googlesitekit-datastore-forms-********************.js	8.78 kB
./dist/assets/js/googlesitekit-datastore-location-********************.js	2.08 kB
./dist/assets/js/googlesitekit-datastore-site-********************.js	14.5 kB
./dist/assets/js/googlesitekit-datastore-ui-********************.js	8.88 kB
./dist/assets/js/googlesitekit-datastore-user-********************.js	21.1 kB
./dist/assets/js/googlesitekit-i18n-********************.js	3.92 kB
./dist/assets/js/googlesitekit-idea-hub-notice-********************.js	45.1 kB
./dist/assets/js/googlesitekit-idea-hub-post-list-********************.js	26.1 kB
./dist/assets/js/googlesitekit-modules-adsense-********************.js	65.9 kB
./dist/assets/js/googlesitekit-modules-analytics-4-********************.js	19 kB
./dist/assets/js/googlesitekit-modules-analytics-********************.js	68.2 kB
./dist/assets/js/googlesitekit-modules-********************.js	19.1 kB
./dist/assets/js/googlesitekit-modules-idea-hub-********************.js	34.7 kB
./dist/assets/js/googlesitekit-modules-optimize-********************.js	19.3 kB
./dist/assets/js/googlesitekit-modules-pagespeed-insights-********************.js	18.3 kB
./dist/assets/js/googlesitekit-modules-search-console-********************.js	38.6 kB
./dist/assets/js/googlesitekit-modules-subscribe-with-google-********************.js	17.3 kB
./dist/assets/js/googlesitekit-modules-tagmanager-********************.js	30 kB
./dist/assets/js/googlesitekit-polyfills-********************.js	380 B
./dist/assets/js/googlesitekit-settings-********************.js	53.6 kB
./dist/assets/js/googlesitekit-user-input-********************.js	49.6 kB
./dist/assets/js/googlesitekit-vendor-********************.js	323 kB
./dist/assets/js/googlesitekit-widgets-********************.js	18.5 kB
./dist/assets/js/googlesitekit-wp-dashboard-********************.js	59.9 kB
./dist/assets/js/runtime-********************.js	1.22 kB

_{compressed-size-action}

[asvinb]

LGTM!

[techanvil]

LGTM!