[ESLint] Support .eslintignore in project root

Is your proposal related to a problem?

ESLint is trying to lint some pre-built files that I dropped into my src directory, but it’s finding “errors”, and causing the build to fail.

Describe the solution you’d like

I want to be able to drop a .eslintignore file in my create-react-app root and have it Just Work ™️.

Describe alternatives you’ve considered

I tried using the EXTENDS_ESLINT env variable and overriding specific rules in my custom src/.eslintrc.js, but overriding each rule one at a time, but it’s easy to miss some. I really just don’t want ESLint even trying to lint the file at all.

Additional context

I’m working on a new version of React Router and it’d be nice if people could just drop react-router.production.js into their src directory and see everything work w/out having to worry about ESLint failing. Google Closure Compiler does some interesting things to our built code that ESLint complains about.

I already submitted #8063 which fixes the main issue, but it’d be nice to just disable ESLint completely on any given file, like you’d be able to in a normal ESLint project.

[ianschmitz]

No problem! Our plan is to make this the default behavior in v4 where it will just work as you say, but for v3 we made it an opt in.

[miraage]

@mjackson I believe there is a typo in the environment variable.