Add .nvmrc + fix node version for eslint 5 compat (#20109)

Summary:
A .nvmrc file is practical when working on many node projects with different version requirements.

Also, the current required node engine is incompatible with eslint5 requirements:

```js
  "engines": {
    "node": ">=8"
  },
```

It produces the following warning on install:

> error [email protected]: The engine "node" is incompatible with this module. Expected version "^6.14.0 || ^8.10.0 || >=9.10.0".
> error Found incompatible module

I made the requirement compatible with eslint5 requirements
Pull Request resolved: https://github.com/facebook/react-native/pull/20109

Differential Revision: D9071440

Pulled By: hramos

fbshipit-source-id: 0d3728d03cad20d22c67a731a6d283cea48b8da8