History of Array.prototype.findIndex()

adds strict mode to prevent getting window obj in 'this' reference when called as function

changed incorrect check `===null` to `==null` to test for `null` *and* `undefined` (even the error that is being thrown by this code states that it is a check for null and undefined)

Add "Return value" section as per Bug 1287421

Remove {{harmony}}; remove "Experimental" tag; add link to ES draft spec

Fixes lint warning due to == comparison instead of ===. Array.prototype.find document was already fixed in the same way by someone else

fix compat parsing error; cleanup

Typo in the method name