[Emotion] Fix multiple css props not being properly cascaded/merged to child props

[cee-chen]

Summary

See elastic/kibana#140706 (comment)

This PR fixes the css prop being passed to various child componentProps not correctly merging css. Apparently Emotion generally requires our css={} definition to come before the {...props} spread for it to be combined correctly.

Or in the case of EuiPageSection, it requires the concatenation to occur in our Emotion styles array declaration (I think because contentProps has no classNames on it).

NOTE:

• ⚠️ This PR does not include components that were converted to Emotion after 64.x ⚠️
  • because this PR needs to be backported to the latest Kibana upgrade, I kept it limited to components converted in 64.0.0 and below
• ⚠️ This PR does not include components that have not yet been converted to Emotion ⚠️
  • I plan on opening up a follow-up PR to add regression Jest tests for those components
• ⚠️ This PR does not include EuiPopover's panelProps ⚠️
  • Because panelProps specifically excludes passing style. Still trying to figure out/think on how to ignore that, and it's getting late + I'm about to go on PTO so I figured I'd get this in sooner rather than later.

Checklist

• Added or updated jest and cypress tests
• A changelog entry exists and is marked appropriately

[cee-chen]

I'll be OOO until afternoon Pacific Time tomorrow. If this looks sane to folks feel free to merge/backport release this into 64.04 for elastic/kibana#140323. Otherwise I can do it when I get back

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_6239/

[1Copenut]

Your PR description is one I'm going to refer back to often. It defined the problem space perfectly, outlining that Emotion CSS must be defined before {...props} or the CSS won't merge correctly.

Commit 5f4af08 really helped me understand the intent.

The PR looks good to me. I'll wait to hear from one more reviewer before merging.

[chandlerprall]

Changes LGTM, pulled and verified the child prop assertion addition to shouldRenderCustomStyles

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_6239/