[Emotion] Memoize all generated styles

[cee-chen]

• [x] Setup (+ EuiCode & EuiHealth): https://github.com/elastic/eui/pull/7529

Highest impact components

• [x] Flex + Spacer: https://github.com/elastic/eui/pull/7582
• [x] Buttons: https://github.com/elastic/eui/pull/7541 (+ #7634)
• [x] Text: https://github.com/elastic/eui/pull/7543
• [x] Icons: https://github.com/elastic/eui/pull/7544
• [x] EuiPanel: https://github.com/elastic/eui/pull/7584

Medium impact components

• [x] EuiHorizontalRule, EuiEmptyPrompt, EuiCallOut, EuiBadge, EuiTabs, EuiDescription, EuiContextMenu: #7638
• [x] Breadcrumbs: https://github.com/elastic/eui/pull/7580
• [x] Skeletons: #7562
• [x] Loading: https://github.com/elastic/eui/pull/7649
• [x] EuiFlyout: https://github.com/elastic/eui/pull/7636
• [ ] EuiModal, EuiAccordion
• [ ] EuiToolTip, EuiIconTip, & EuiPopover
• [ ] EuiPage+
• [ ] EuiCard

Low impact components

• [x] Toast: https://github.com/elastic/eui/pull/7568
• [x] EuiResizableContainer: https://github.com/elastic/eui/pull/7584
• All the rest (TODO)
• [x] https://github.com/elastic/eui/pull/7635
• [x] https://github.com/elastic/eui/pull/7637
• [ ] F-L
• [ ] L-S
• [ ] T-Z

[github-actions[bot]]

👋 Hi there - this issue hasn't had any activity in 6 months. If the EUI team has not explicitly expressed that this is something on our roadmap, it's unlikely that we'll pick this issue up. We would sincerely appreciate a PR/community contribution if this is something that matters to you! If not, and there is no further activity on this issue for another 6 months (i.e. it's stale for over a year), the issue will be auto-closed.