feat: support for ESLint v9

[Copilot]

Upgrades @readme/eslint-config and eslint-plugin-readme to ESLint v9 exclusively (v9.39.1).

Changes

Package dependencies:

• Updated peerDependencies to ^9.0.0 (ESLint 8 support removed)
• Moved ESLint dependencies from root package.json to eslint-config and eslint-plugin packages:
  • eslint now in devDependencies of eslint-config and eslint-plugin
  • @eslint/eslintrc and @eslint/js distributed to packages that need them
  • eslint-plugin-perfectionist moved from root to eslint-config where it's used
• Replaced deprecated eslint-plugin-node with eslint-plugin-n v17

Configuration migration:

• Created eslint.config.js files for eslint-config and eslint-plugin using FlatCompat to support ESLint 9's flat config format
• Updated rule references from node/* to n/* throughout config files

Test infrastructure:

• Migrated RuleTester configurations to flat config format:

  // Before (eslintrc format)
  new RuleTester({
    parser: require.resolve('@typescript-eslint/parser'),
    parserOptions: { ecmaVersion: 2022 }
  })
  
  // After (flat config format)
  new RuleTester({
    languageOptions: {
      parser: require('@typescript-eslint/parser'),
      ecmaVersion: 2022,
      sourceType: 'module',
    }
  })

• Removed redundant output properties from test cases (ESLint 9 requirement)

Branch synchronization:

• Merged latest main branch which migrated spectral-config and stylelint-config from ESLint to Biome
• These packages now use Biome for linting instead of ESLint

The packages now require ESLint 9 exclusively. Root package.json no longer contains ESLint dependencies as they've been moved to eslint-config and eslint-plugin packages. Note that spectral-config and stylelint-config are no longer using ESLint as they've been migrated to Biome.

Original prompt

Upgrade the eslint-config and eslint-plugin packages to use eslint v9

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[erunion]

@copilot remove support for eslint 8 and move the eslint dependencies in the root into the eslint-config and eslint-plugin packages

[Copilot]

@copilot remove support for eslint 8 and move the eslint dependencies in the root into the eslint-config and eslint-plugin packages

Done! Removed ESLint 8 support (peerDependencies now require ^9.0.0) and moved all ESLint dependencies from root to individual packages. Changes in commit 3245aff.

[erunion]

@copilot update this branch with the latest main

[Copilot]

@copilot update this branch with the latest main

Done! Merged main branch which included the Biome migration for spectral-config and stylelint-config. Resolved conflicts by removing ESLint configs from those packages. Changes in commit f431f66.