Relates to #4171

Summary

Having been granted access to Windsurf, I wanted to put it to the test. I consider this PR a reference for what it can do and for a future PR toward a new split button.

TODO

• Add 1px divider/spacer to text + fill version

Screenshots

• Under the hood this uses an EuiButton and EuiButtonIcon wrapped in an EuiPopover.
• Color, size, and fill are enforced to be consistent.
• Storybook and test were also created with Windsurf.

Dark mode, text color with border style

Light mode, filled, primary color with 1px gap

General checklist

• Browser QA
  • Checked in both light and dark modes
  • Checked in both MacOS and Windows high contrast modes
    • (emulate forced colors if you do not have access to a Windows machine.)
  • Checked in mobile
  • Checked in Chrome, Safari, Edge, and Firefox
  • Checked for accessibility including keyboard-only and screenreader modes
• Docs site QA
  • Added documentation
  • Props have proper autodocs (using @default if default values are missing) and playground toggles
  • Checked Code Sandbox works for any docs examples
• Code quality checklist
  • Added or updated jest and cypress tests
  • Updated visual regression tests
• Release checklist
  • A changelog entry exists and is marked appropriately.
  • If applicable, added the breaking change issue label (and filled out the breaking change checklist)
• Designer checklist
  • If applicable, file an issue to update EUI's Figma library with any corresponding UI changes. (This is an internal repo, if you are external to Elastic, ask a maintainer to submit this request)