Detect .flowconfig and automatically enable ESLint rule to enforce Flow headers

Created by: FezVrasta

Is your proposal related to a problem?

Right now I need to use customize-cra just to extend the react-app ESLint preset with the following rule:

rules: {
  'flowtype/require-valid-file-annotation': [
    'warn',
    'always',
  ],
},

which, basically, forces every file to have a // @flow (or // @noflow) comment on top of them.

Describe the solution you'd like

CRA should check for the presence of the .flowconfig file and automatically enable the Flow-related ESLint rule.

Describe alternatives you've considered

Alternatively, following the --typescript flag approach, provide a --flow flag that can be used to initialize the project with the above Flow-related rule enabled.

Additional context

I'm willing to work on a PR to introduce this if you are interested. Actually, I just prepared it since it was quick to do: https://github.com/facebook/create-react-app/pull/7547