Export select option type

[mattkime]

Summary

It would be helpful to be able to import the Option type into my kibana code. This is partial work as I'm not sure how what I'm attempting here works with the typedef files. A small nudge should help it across the line.

Checklist

• Check against all themes for compatibility in both light and dark modes
• Checked in mobile
• Checked in IE11 and Firefox
• Props have proper autodocs
• Added documentation examples
• Added or updated jest tests
• Checked for breaking changes and labeled appropriately
• Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[chandlerprall]

Also needs a CHANGELOG.md entry, like

eui/CHANGELOG.md

Line 9 in c2eb8d9

- Converted `EuiDescribedFormGroup` to TypeScript ([#2810](https://github.com/elastic/eui/pull/2810))

[thompsongl]

jenkins test this

[chandlerprall]

LGTM; pulled & tested locally, including eui.d.ts build to confirm the type is exported.