Jm/eslint formatting

[JonathanMatthey]

Changes

• Added ESLint dependencies (eslint, @typescript-eslint/*, eslint-plugin-astro, astro-eslint-parser)
• Created .eslintrc.cjs with TypeScript and Astro support
• Added scripts: lint, lint:fix, format
• Created .eslintignore to exclude build artifacts and dependencies
• Updated README with formatting setup and usage instructions
• Added/updated PR template with formatting checklist
• Added CI workflow to enforce ESLint checks on PRs