chore: migrate dev dependency ESLint to v9

[Belco90]

Checks

• I have read the contributing guidelines.

Changes

• Upgrade ESLint ecosystem to be compatible with ESLint v9 and flat config
• Migrate config to flat format
• Apply other improvements:
  • Enforce consistent type import, so they are separate imports and are in the last position of import groups
  • Remove specific tsconfig for linting, in favor of reusing the base tsconfig to avoid duplication
  • Replace eslint-plugin-import with eslint-plugin-import-x
  • Update lint-staged command

Context

N/A