[Workstream] New UI design

# Summary

A broad set of changes proposed as the "New UI design".

Engineering work captured here as it is moves toward ready for dev.

Lead by: @ryankeairns and Platform UX
Source of truth: [[META] New UI design - component defaults #902](https://github.com/elastic/platform-ux-team/issues/902)

# Planned changes

## December

These changes will support the Shared UX New Chrome initiative, tracked [here](https://docs.google.com/document/d/1hqNPXWYL3kHnbYStlTcYfa6ot2fskZN9CyBqxByRGN0/edit?tab=t.0#heading=h.c6dlesj2d6z3) (elastic internal link).

(Mirrored in [[Meta][New UI Design] Improvements for 9.3](https://github.com/elastic/eui/issues/9188))
- [ ] https://github.com/elastic/eui/issues/9186
- [ ] https://github.com/elastic/eui/issues/9187
- [ ] https://github.com/elastic/eui/issues/9184

## Not yet planned
- [x] https://github.com/elastic/eui/issues/9185
- [ ] https://github.com/elastic/eui/issues/9189