[Emotion] Memoize EuiSkeleton components

[cee-chen]

Summary

Pulled out the original work/impetus from Kevin's PR (#7374) for PR atomic-ness and also because it dovetails nicely into our existing Emotion perf/memoization work (see #7561)

QA

• Staging should look the same as production

General checklist

• Browser QA
  • Checked in both light and dark modes
  • Checked for accessibility including keyboard-only and screenreader modes
  • Checked in Chrome, Safari, Edge, and Firefox
    - [ ] Checked in mobile
• Docs site QA - N/A
• Code quality checklist - N/A
• Release checklist - Skipping as this should not impact consumers or end users
• Designer checklist - N/A

[kibanamachine]

Preview staging links for this PR:

• Docs site: https://eui.elastic.co/pr_7562/
• Storybook: https://eui.elastic.co/pr_7562/storybook

[elasticmachine]

💚 Build Succeeded

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• Commit: 9d405fe