chore(eslint): migrate to eslint 9 and flat file configs

[hanneskuettner]

Summary

Migrate ESLint from v8 (legacy .eslintrc.js) to v9 flat config with co-located package config and make use of a single workspace eslint.config.js.

Before this PR it looks like no lint actually ran?

Changes:

• Upgrade ESLint 8 to 9 with flat config format
• Remove the @dnd-kit/eslint-config package (configs were broken/missing which might be the reason why no proper lint ran)
• Add co-located eslint.config.js files in packages that need custom rules
  • Since previously no linting took place I have started with the most relaxed configurations possible to not include additional noise in this PR and make sure the lint passes as-is. If we want we can tackle a couple of the currently ignored rules and make some improvements
• Create shared React config at config/eslint/react.config.js for reuse across react and stories package
• Move eslint plugins to package.json files where they're actually used and remove eslint from individual packages
• Use --flag v10_config_lookup_from_file for proper config resolution per package (this can be removed as soon as it stabilized in eslint v10

Structure:

• Root config: strict TypeScript rules shared by all packages
• Package configs: extend root + add package-specific relaxations as warnings
• Framework configs: pure (don't import root), composed by packages as needed (will be able to add vue, svelte etc)

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Latest commit: f02b131

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commit: f02b131

[hanneskuettner]

Haven't worked with bun a lot, is this normal that these are bumped now in the lock file? I would have expected them to use the workspace protocol and automatically get back-filled on release?