chore: lint TypeScript declaration files d.ts

[MikeMcC399]

Situation

• PR feat: add type declaration file #279 added a TypeScript declaration file lib/flat.d.ts

• The ESLint configuration eslint.config.mjs only lints JavaScript files

Change

• Update ESLint-related devDependencies packages to latest versions

• Add @eslint/js explicitly

• Add typescript and typescript-eslint as devDependencies (see typescript-eslint documentation)

• Expand eslint.config.mjs to lint *.d.ts and use recommended rules

Verification

npm ci
npm test
npm run format:check
npm run lint

[cypress-app-bot]

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