What is the purpose of this pull request? (put an "X" next to item)

[X] Changes an existing rule #12207

Examples of incorrect code for this rule with "enforceForIndexOf" option set to true:

/*eslint use-isnan: ["error", {"enforceForIndexOf": true}]*/

var hasNaN = myArray.indexOf(NaN) >= 0;

var firstIndex = myArray.indexOf(NaN);

var lastIndex = myArray.lastIndexOf(NaN);

What changes did you make? (Give an overview)

New option enforceForIndexOf in the use-isnan rule.

Is there anything you'd like reviewers to focus on?

It seemed too complex to have an advice in the message since it depends on whether the user wants to find index or just check for existence, and also what environments are available. There are examples in the docs for different environments.