[new] Add prefer-read-only-props rule

[drx]

This adds a rule to enforce that flow types for props are read-only (covariant). This was requested in #1454 and #1869.

I added a fixer for the rule, which I tested on a decently large codebase at Flexport (5000+ files) with thousands of violations.

One interesting note is that declaredPropTypes doesn't recognize foo in props: $ReadOnly<{foo: bar}> as a prop. In this case that's OK, because that wouldn't give a warning anyway.

Fixes #1454. Fixes #1869.

[ljharb]

It'd be great if we could separate out the fixes into a separate PR from the new rule.

[alexzherdev]

So cool to see the propTypes detection in use 👍

[alexzherdev]

@drx to clarify this bit:

Worse yet, because declaredPropTypes is an object, those helper properties can overwrite actual props.

Do you mean that if we wrongly detect something as a prop, it can overwrite the metadata for an actual prop with the same name?

[drx]

It'd be great if we could separate out the fixes into a separate PR from the new rule.

Done! #2111

@alexzherdev Yeah, if you have something like:

type Props = {
  foo: string,
}

class Hello extends React.Component<Props> {
  renderHelper = ({foo}: {foo: string}) => {
    return <div />;
  }
  render() {
    return <div>{this.props.used}</div>;
  }
}

The metadata (including the loc) might be overwritten by the renderHelper foo, I have had this happen while running fixes which is how I discovered this.

[ljharb]

Looks great!

(I'm going to hold off merging it for awhile while the 7.12 release settles down)

[drx]

Thanks!

[EvHaus]

LGTM. I'm looking forward to using this one.