[restrict-plus-operands] False positive when using Extract and generics

[boris-petrov]

I'm using the latest version of all packages (1.6.0).

function x<T>(z: Extract<keyof T, string>): string {
  return z + '__';
}

This warns that the addition is unsafe but it shouldn't. It works correctly if it's not keyof T but keyof obj where obj is some object, i.e. generics are not involved.

[bradzacher]

Fixed in #440