`eqeqeq`: allow `null`/`undefined` checks to simplify `NonNullable` checks

[csvn]

Using eqeq (==) in TS/JS is almost always best to avoid. But there is one use-case which is very legitimate, and that is checking for both null + undefined. I think the only thing blocking this rule from being included in recommended is this case.

function fn<T>(value: T) {
  // Replacing `undefined` with `null` works the same
  if (value != undefined) { 
    value satisfies NonNullable<T>;
  }
}

The above code is a slightly more terse but equivalent way of writing out two checks with ===:

function fn<T>(value: T) {
  if (value !== undefined && value !== null) { 
    value satisfies NonNullable<T>;
  }
}

Any other == usage is more likely to introduce bugs or unexpected behavior due to type conversion that is often hard to understand when comparing different types of values.

If this is fixed, I think it would make it very reasonable to add eqeqeq to the recommended tag.

Suggested valid code

Comparing a variable against null or undefined.

a == null;
a == undefined;
a != null;
a != undefined;

Suggested invalid code

Keep all other == + null/undefined expressions as invalid.

true == undefined;
123 != null;
// etc