fix eslint

[gurgunday]

I'm getting the following error in every JS file's first line:

Parsing error: ESLint was configured to run on `<tsconfigRootDir>/index.js` using `parserOptions.project`: .../fastify-multipart/tsconfig.eslint.json
However, that TSConfig does not include this file.

As a result no actual static checking happens

This PR simplifies the config and fixes that issue

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm