feat: remove dependency to @emotion/react + adopt native css solution

[arjunvegda]

Summary

• Migrate Mantine to v7 and adopt full native CSS solution
• Fixes #104
• Supercedes #126

Remaining tasks

• [x] Come to a resolution on the Mantine theme issue and document it in the PR description
• [x] Test on NextJS
• [x] Test on Vite
• [x] Update readme.md with a migration guide
• [x] Update jotai.org docs (PR: https://github.com/pmndrs/jotai/pull/2506)

⚠️ Warning for Mantine users

• Using Jotai DevTools with Mantine on userland causes issues with theming on DevTools. For example, when you change the color scheme on userland to from light to dark, it will toggle Jotai DevTools' color scheme too. However, not fully. Some elements remain in light mode
• Mantine's team is open to fixing this problem but its complex. You're welcome to take a stab at it
• Temporary remedy for the users of Jotai DevTools is to sync their theme via props by passing computed color scheme value via props.theme

Contributors

Huge thanks to these folks for contributing this change to Jotai DevTools 🙏 @speaker-ender (https://github.com/jotaijs/jotai-devtools/pull/126), @WebDevMcFarland, @tori-obregon, @adamkim1006 and more!

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