Migrate to ESLint flat config

[Copilot]

Moves from the legacy .eslintrc.json format to ESLint's flat config format (eslint.config.js), required since ESLint v9.

Changes

• eslint bumped ^8.57.0 → ^10.9.1
• typescript-eslint added — the canonical flat-config wrapper that exposes tseslint.config() and replaces direct parser/plugin wiring
• eslint.config.js created — replaces .eslintrc.json with equivalent rules, per-glob overrides, and eslint-config-prettier integration
• .eslintrc.json removed
• .eslintignore removed — patterns moved into the ignores array in eslint.config.js

// eslint.config.js (flat config)
export default tseslint.config(
  { ignores: ['.dev/', 'dist/', 'integration-results/', 'node_modules/', 'src/.observablehq/'] },
  tseslint.configs.strictTypeChecked,
  tseslint.configs.stylisticTypeChecked,
  importPlugin.flatConfigs.recommended,
  importPlugin.flatConfigs.typescript,
  ...markdown.configs.recommended,
  eslintConfigPrettier,
  { /* custom rules */ },
  { files: ['**/*.js'], extends: [tseslint.configs.disableTypeChecked] },
  // …per-glob overrides
)