More aria attributes for Selectable

[myasonik]

Summary (3 things)

1. Adds aria-setsize and aria-posinset attributes to the options (gives accurate sizes and positions to screen readers in virtualized lists)
2. Adds the messages (e.g., "no results found") as an aria-describedby to the search field
3. When clicking on an option, it would set focus on the first focusable option. Now, instead, it sets it on the option you clicked which prevents a weird jump and seems to just work better. (Breaking change: needed to pass around another function for this)

Reviewer note: Each of these is a single commit if you want to review that way.

Checklist

• Checked for breaking changes and labeled appropriately
• Checked for accessibility including keyboard-only and screenreader

- [ ] 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
- [ ] Checked Code Sandbox works for the any docs examples
- [ ] Added or updated jest tests
- [ ] A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3583/

[thompsongl]

LGTM!

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3583/