Add ESLint 9 support and migrate to flat config

[Copilot]

Migrates to ESLint 9 and flat config format. ESLint 8 reached EOL on October 5, 2024 and traditional configuration files are deprecated.

Breaking changes - consumers must migrate their configuration files manually.

Changes

• ESLint 9 upgrade: Updated all ESLint-related dependencies to ESLint 9 compatible versions
• Flat config migration: Converted all configurations to the new flat config format
  • configs/base.js, configs/typescript.js, configs/prettier.js now export config arrays
  • Uses projectService: true for TypeScript parser (recommended for ESLint 9+)
  • Uses eslint-plugin-prettier/recommended for flat config integration
• Updated exports: Main entry point and plugin adapted for flat config consumption

Usage

// eslint.config.js
const wpvip = require('@automattic/eslint-config-wpvip');

module.exports = [
  ...wpvip.configs.base,
  ...wpvip.configs.typescript,
  ...wpvip.configs.prettier,
];

Release plan

Publish as 1.0.0-rc@next for testing before stable 1.0.0 release.

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[sjinks]

@copilot I don't see any changed files. Please make sure you have committed your changes.

[Copilot]

@copilot I don't see any changed files. Please make sure you have committed your changes.

My previous interaction only involved responding to automated review comments - no code changes were needed since the implementation was already correct. All the ESLint 9 migration work is in commit 9cf21a6. The empty commit (0109dd9) was created inadvertently and can be ignored or removed.