boardgameio / boardgame.io / 31389618222

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Migrate ESLint to flat config so ESLint 9 can land (#1325)

Replace `.eslintrc` and `.eslintignore` with a single `eslint.config.js`,
porting the existing extends, plugins, parser, env, rules and settings to
their flat-config equivalents at the already-installed plugin versions:

- `eslint:recommended` -> `@eslint/js` `js.configs.recommended`
- `plugin:jest/recommended` -> `configs['flat/recommended']`
- `plugin:unicorn/recommended` -> `configs['flat/recommended']`
- `plugin:react/recommended` -> `configs.flat.recommended`
- `plugin:@typescript-eslint/recommended` -> `configs['flat/recommended']`
- `plugin:prettier/recommended` -> `eslint-plugin-prettier/recommended`

`env: { node, browser, es6 }` becomes `languageOptions.globals`, and the
seven `.eslintignore` entries become an `ignores` block.

Bump `eslint` to ^9, promote `@eslint/js` to a direct devDependency, and
add `globals`.

The migration is behaviour-preserving -- `pnpm run lint` reports the same
(zero) problems before and after across the same files:

- Flat config lints dotfiles that `.eslintrc` skipped by default, so the
  jsdom stub `.empty_module.js` is added to `ignores` to stay unlinted.
- Flat config defaults `reportUnusedDisableDirectives` to "warn"; pin it
  back to "off" to match the previous default.
- Drop the now-unsupported `/* eslint-env es2020 */` comment from
  `jest.setup.js`; its globals are covered by `languageOptions.globals`.

`pnpm run lint` and `pnpm test` (43 suites / 903 passed) are green on
ESLint 9.

2038 of 2038 branches covered (100.0%)

Branch coverage included in aggregate %.

2918 of 2918 relevant lines covered (100.0%)

26332.79 hits per line